Will stays behind.

"Complain if you want to", Will continues the argument, "but you better get the swimmin' while the swimmin's good!"
Allison looks at Will, realizing there's no reasoning with him when he sets his mind. He has already focused all of his attention at the now smoking beginning of his fire.
" Will," she says, whining a little. "I got no swimsuit."
"You can find something, Allison" he says, not looking up at her.
After rummaging through her things she settles on a black ribbed tank top and some baggy cargo shorts, it's more-or-less her pajamas. Opting against bra and panties as they will take too long to dry, and she really doesn't want to put them by the fire to do so. Musketeers can razz you for your undies like anyone else!

Shoeless, she heads out for the lake, wondering if shoes would work better for the rocky shore, and eventually thinking better of it, one more thing to dry or keep track of. God, she hates the outdoors! Marcus is out about 30 feet splashing around and diving. She hollars out "You look like a dweeb out there!"
He turns to look at her and seems stunned for a moment, deer in the headlights type of look on his face. He smiles after a second.
"You won't believe how warm the water is!" He yells back, then dives off farther away.
Great, Allison thinks, he thinks it's some kind of race or something. Warm, huh? She dips her toe in, and it's instantly frozen! Must be some kind of Glacial runoff! Frowning, she wades in up to her knees and jumps in the rest of the way. In for a penny, in for a pound, that's what dad used to say, the goofy bastard! She gets a little aclimated to it, but it's still damn cold. A feeling comes over her, a feeling of protectiveness, a maternal instinct. She suddenly looks out to where Marcus is swimming and bolts out to him. She gets to him, and the water is only about waist high. She puts her feet down and addresses her friend, raising an accusatory finger.

~Marcus swims away from Allison, maybe this crazy boner will die down before she decides to come over. He doesn't want Allison to see it any more than he would want his own mom walking in to a jerking session! The water still feels good, he'll swim away and she'll probably stay over near the shore. Swimming around is pretty awesome, the water still feels sorta warm, and he feels like he's drunk or something, cloudy. He feels excited!
He turns around and, while he had basked in the feeling, Allison had swam up to him, and was right up in his grill! Not only that, though, the cold water has affected her nipples, they're hard as rocks! Her tank top soaked, her ample chest (courtesy of her chubbiness) is completely visible with the clinging thin undershirt. He struggles to look her in the face.

~Where does he think he is? The local Y? Swimming so far from shore, what if something should happen? She lifts her finger up to just below his chin, pointing at the middle of his chest.
"Do you think you're invincible? This is a huge lake, you need to stay closer to the shore!" she says, keeping her eyes locked on his distracted face.
"You're not my mom! I'll be fine!" He replies, not quite angrily, squirming (under her gaze, or something else?)
He backs up from her into shallower water, hands out to fend off her tirade.
" I have every right to be concerned about you swimming with cliffs and rocks in this Lake that god-knows-how Will found and-Is that your dick!?!" She says, looking down at his fully engorged member, clearly visible through his wet jockeys. God! She's pretty sure she can SEE it throbbing with his pulse, it's so completely erect. He's also blushing, she imagines she is too. Her lambasting ends mid-sentence.
"Yeah, your boobs aren't helping with it either." Marcus says. Barely holding his composure. This is so wierd, Allison thinks, she's never been the one Marcus gawked at, and that's fine, they're just friends!
"Well, take care of that later in the tent or something." Allison says unconvincingly, her cheeks burning, glancing down at her bust with her pert stiff nipples.

"C'mon, let's get back to camp." She continues, he is trying to fold his body to hide the erection for her benefit, but there's just no doing it. She similarly tries to help by covering her top but only succeeds in mashing her breasts to her body in order to cover her nipples. She stumbles a bit due to the rocky bottom of the lake, headed closer to shore. Her head aflutter with embarassment, she keeps treading closer to the land. Marcus, to his credit, is still trying to keep his distance and avert his eyes, 'poor thing' she thinks, (where did THAT come from!) she feels sorry for him, it must be tough.

"Almost out of the water, bud!"She says, trying to put emphasis on the 'bud' part. Not really all that sure it's working. Allison comes up on the shore, and rushes to get on land, almost immediately regretting it.
The cargo shorts, not made for swimming, have taken on water in the large pockets. The added weight causes the shorts to sag. Now, in her **** attempt to get out of a compromising position with her friend, following close behind her, she has instead escaped the water so her shorts drop so low that her ample bottom is hanging bare to him, not 5 feet behind. Her pale derierre exposed to him as he tries not to think about her fat tits and rock hard nipples from moments ago. Her poor boy, he IS hers, they ARE both her responsibility, she's felt this way since stepping into the lake with him.

~Self-control, Marcus, he tells himself, following Allison out of the water. She's right, he'll sneak off into the tent as soon as they get back, and think about those giggly breasts and the wet shirt and the hard nipples and- His cock twitched hard- Self control, Dammit! She's headed out of the lake, right, he'll follow. Then take care of his business. She starts to speed up, aiming to leave the water. Oh, my, god! Marcus thinks. Her shorts, leaden with water, fall down revealing her fine, round, pale, ass. She realizes it and immediately it clenches. The muscles forming little oblong ovals at the top, the crease between the back of her thigh and the buttock pronounces. She looks over her shoulder, worry plays across her face, and then something like pity as she looks down at his cock and it twitches again.
"Umm." she sputters, her eyes flashing three times down to his crotch. "Well." She continues, and nervously licks her lips. She slowly reaches down and pulls up the sagging garment.
Her cheeks are fully flushed, and her eyes seem unable to keep on his face, soon she is staring at his dick, no longer with the pretense of ignoring it. She reaches down and touches it. They both gasp in unison, and beneath the underwear Marcus' cock jerks, pre-cum oozing out of the tip. She pulls his jockeys down with one hand, still absently holding her shorts up with the other. Marcus puts a hand out and firmly pushes down on her shoulder, forcing her to ease down onto her knees, face level with his hard cock. She smells the musky scent of his crotch, looking at his stiff dick it throbs in front of her. She looks up at him, his strained expression protraying how difficult this is for him, and how URGENT! Marcus looks down at Allison, she is looking up at him her brown eyes soft as they examine his face, beyond her head and down her back the heavy shorts are again sagging, revealing the crack of her ass, pasty white and wet. She opens her mouth and takes in the head, he grunts and holds back the thrust he wants to gag her hot mouth with. She starts to lick the underside of his shaft and end each lick with a slurpy suck on the head of his cock. His hands go to either side of her head and he begins to guide her down onto the shaft, forcing more in, little by little. She starts to gag, and slimy spit provides some needed lubrication to mix with his flowing pre-cum.

After a minute or two, he knows he can't hold out much longer. He lifts her up to her feet before spinning her around and bending her over at the waist. Grabbing a handfull of her left boob, he uses his right hand to yank her sagging shorts down. licking the fingers on his right hand, he reaches down to find her wetness. There seems to be no need to lick the fingers here. She is slippery with anticipation. He thrusts into her and begins pounding, they fall to the shore together and flatten some he still does not slacken his pace. She begins to huff and squeak with his thrusts, fueling his frenzied **** on her. His grunts become more harsh as he speeds to his climax, and the two of them share a muffled cry as he empties his semen into her prone form beneath him and she shudders from an aftershock of her intense orgasm.

Now, your move again:
Do they go back and tell Will what happened.

Do they keep the secret, not knowing how Will would react.