My dolphins keep drowning themselves

I'm building a Minecraft zoo I want to include almost every animal in Minecraft in it with some restrictions (for example villagers and enderman)

So I built an aquarium but, my dolphins keep drowning themselves I've tried everything I could think of having less water having fewer plants having the ground incline instead of being flat, but no matter what I do the dolphins choose to drown themselves. I know that this has already been reported twice but it says the issues have been resolved, but it is still a problem for me.