Have to admit, I’ve tried MF sex in a bath, jacuzzi and sea and been a bit underwhelmed with all of it. I think the body of water gets in the way.
Shower sex however is a mainstay. Not only do I get to cum, I get to have the cleanest boobs in the world (because he takes charge of washing them) 
We both know we’d fall and break something. LOL so we “wash” each other and hold off till we get out and on the bed, or couch or lounge outside.
We are to big to both fit in the bath at the same time but have often fingered her in the bath and played with her boobs. We stayed in a hotel with a hot tub and that was fun.
Changed our bath in a recent refit and both of us can no longer get in together. 