More underwater/above water gameplay features/items include:
- Swimming in ponds including fishing and backyard ponds used with water tool
- Swim up bars and cafes in pools and grottos (like you said)
- Special foods that can be created using ingredients only found underwater (especially easy for mermaids)
- toys for kids they throw in the water and dive down to find
- New events like algae blooms that need to be cleaned, and sometimes toxic algae bloom events that can result in death if swam in
- Fishing and shelling jobs in addition to your marine conservationist idea
- more interactions like watching bioluminescent phytoplankton, whale watching, and shell collecting
- Buildings that are completely submerged in water and only accessible to mermaids and those with scuba gear in addition to your grotto idea