One seated sex position that makes excellent use of a shower bench or seat is the wet lap dance sex position.
The partner doing the penetrating sits on the edge of the ledge, while the person doing the receiving sits on their lap. To get a better thigh exercise in this position, try shifting your weight to your penetrating partner's lap. Because it frees up both partners' hands to explore, this position is awesome!
Savoury, entertaining, and, uh, functional shower sex! Having sex on a wet, slippery floor can be risky, so either invest in a non-slip shower mat or take extra precautions to avoid slipping.
Attach sex devices here if you like; just make sure they're watertight!