Drowned - zombie like mob with grey-blue skin. It can't swim, but has immunity to suffocation damage so can't die in water. It should spawns in rivers, lakes and swamps biomes. At attacking it should apply new potion effect - "Breathlessness" - you'll started losing air.
Phantom Diver - similar to Drowned mob. Spawns at beach and oceans biomes. Has a rare drop - "Diving Helmet". This helmet should gives unlimited breath in water. https://nightkosh.deviantart.com/art/Phantom-Diver-Concept-185725122

TODO

• Drowned
• Phantom Diver
• Breathlessness
• Diving Helmet

TODO:

• Piece of Diving Suit drop