So dolphins in bedrock dont seem to identify an obstacle in front of them which leads to drowning.  Which is very upsetting because im building a huge habitat right now andd it sucks seeing them drown. If you want to recreate the environment then place dolphins in a large enclosed area and have some form of tall obstacle in the middle of it you might need more than one obstacle in the water. Also multiple dolphins are a good way to get the result faster. In my case they seem to get stuck in either coral or the glass structure as seen below.