Minetest mod to disable drowning, so you can swim and live underwater forever.
Air/breathing limits are a timer that prevents thinking, planning and generally having fun. Underwater effectively becomes a desert that people avoid where possible. Water should be interesting, not gameplay mud.
Minetest already does underwater a lot better than Minecraft traditionally has. You're not blind and mining speed is not reduced. I think underwater would be more interesting if so many people did not grow up with Minecraft teaching them that it's punishment.