Me and my brother were building a "demise" water tank from Hermitcraft S6 because we were talking about demise (we're huge hermitcraft fans). Then I just spawned in some dolphins for fun, without the magma blocks at the bottom, and we turned it into an aquarium-like pool. We put coral and cobblestone pillars and ocean monuments and shipwrecks and ice bergs and so much more! Then all of a sudden all the dolphins started suffocating, and they weren't coming up for water! They all drowned to death.

Steps to reproduce: 

1. Make a bedrock or cobblestone tank (actually it doesn't matter, we just did it that way, but use prismarine and sea lanterns or dark prismarine and blue ice to make it pretty.)
2. Put water at the top, and the water will flow down to fill the pools.
3. Spawn in some dolphins and they might suffocate after like 5 minutes.