DRAGON QUEST BUILDERS 2

In the first area after the crash, collect at least 25 of the kelp laying around. Go stand on a cube of sand that is one block underwater. Once your health gets low just start smashing kelp till it pops.

I got this achievement along the way of the story in Moonbrooke. At some point late in the island's story you'll have to go to an area where you have to go through an underwater tunnel. The path alone will take you about a minute to get to the end, and once you're finished, you can just hang around there for another minute or so for the achievement to pop. Clams that provide air bubbles are nearby too if you don't want to use healing items to stay underwater. When you're done you can just warp back to the castle.

I wasn't able to confirm if using air bubbles reset the timer, but I'd recommend waiting until the long underwater cave at the end of Moonbrooke island's story for this achievement. Have some food or healing items, and wait out the achievement. From the time it took me to get the achievement, I believe the bubbles reset the timer, so you will likely need to outheal the damage.