Take the lady up on her offer.

You start off by groping her shapely ass a little, squeezing the soft flesh and watching as it jiggles. You give her a light spank, and she lightly moans.

You move a hand down to her pussy, feeling for her clitoris and rubbing it with her finger. Pulling it away, it comes off wet and sticky, and you lick up her juices before bringing your hand back to her.

Using your thumb and index fingers, you spread her labia and outer lips, then push in your other three. You push them in and out as her panting grows in intensity.

"You like that?"

"Mmph- mm-hmm! Ah... hah..."

You continue pumping them in and out as your cock grows harder, leaking precum from its tip. Giving in to temptation, you take your fingers out. She whines a little, but stops when she feels your tip on her lips.

"Gonna fuck me? Well, don't half-ass it then..."

You push, and the tip slides in. You bend over her, grunting and moaning alongside her as you get ready to push in.

"Hahh... hahh... haaFUUUUCK!"

Your entire length slides in all at once, seeing as both of you are both producing sex fluids and are also covered in water. You're buried to the hilt inside of her, both of you recovering from your sudden thrust. Your knees are growing weaker by the second, but she grinds against you, letting your dick slosh around inside of her pussy.

"It's my *pant*... safe day *pant*... go nuts."

You eventually do recover, and then pull out, inch by inch until you're almost out. You then thrust hard, pull out, rinse and repeat. Each thrust you're a little faster and a little harder, and gods is it pleasurable.

In a few minutes, both of your moans have reached a crescendo (which of course goes unnoticed by the other bathers). You're close to cumming, and with the bucking of her hips, so is she. She's humping and grinding against your huge cock, spurning you on to orgasm faster. Your dick's the hardest it's been.

You cum hard, shooting rope after rope of thick, bioluminescent semen into her snatch. Her belly bulges slightly from all the cum you've shot into her, with some leaking out onto the bath's floors.

You pull out and both of you collapse onto the floor, tired and spent. Your cock, covered from tip to balls in your glowing cum, starts going flaccid as she begins to wash you. "Least I can do" she says. Not gonna argue with a free wash, though, especially from a pretty lady like her.