My DD's favourite shower/bath toys are containers for holding and pouring water (we use a stainless steel thermos cup lid) and her wooden whale that her dad made for her first birthday to encourage her to bathe 'let's give the whale a swim!'. Of course, wood boats are a classic as well .
Yeah, we have a basket of shells, round river rocks and small pieces of driftwood. My kids love big clam shells for filling and pouring. We also found a few cute cotton terrycloth starfish we throw in.