Unlike Zoro and Nami, Sanji is more agreeable to having his personal space invaded, as long as it doesn't happen when he's cooking something particularly delicate and at risk of getting ruined. Now, here? He welcomes any touch with open arms and hands that tingle with the anticipation of reciprocating. 

"Flaterer. But we should try and keep it down, or one day Nami will throw us overboard." Not like Sanji will insist a lot on self-restraint. Luffy might not know the concept, and the cook likes it when he can make him feel good, so he gets carried away as well.

If the water here were a real risk, Sanji would insist less on shared showers, and he would hold Luffy close to him even more to make sure his captain felt safe and cared for. 

He hums in contempt and tilts his head back a little, giving Luffy's mouth more room to kiss or bite if he feels like it. Meanwhile, one of Sanji's own hands slides down the curve of Luffy's back, giving his hip a squeeze, and presses a thigh in between them. 

"I like it when you say my name." His other hand moves up so he can bury his fingers in those dark curls, massaging the scalp as promised.