Edited for clarity:
Dolphins take no action to avoid drowning, and do not swim up to the surface when they run out of air.
Edited for clarity:
Dolphins take no action to avoid drowning, and do not swim up to the surface when they run out of air.
Community Consensus
Minecraft 18w15a, Minecraft 18w16a
Minecraft 18w19a
Retrieved
Works as intended. Dolphins need air to survive. They can drown, and die in and outside water.