The Update 1.4.0: Update Aquatic - Phase One Title update for Minecraft (Nintendo Switch) has 8 achievements worth 200 gamerscore.
Eat nothing but dried kelp for three in-game days
Power a furnace with a kelp block
Dig up a buried treasure
Find a shipwreck
Find an underwater ruin
Visit all ocean biomes
Collect a fish in a bucket
Spend a day underwater.