AKVALAB takes FIST into the ocean with new traits and equipment tailor made for sub-aquatic missions. Along with that supplemental material that can easily used in any game, AKVALAB also includes a mission so you can test out your gills and spearguns right away.
The mission is moody and offers tons of opportunities for players to take control of the narrative and make it their own. I can see the mission playing out very differently every time, which is always fun.
If you want to add an interesting biome or just want to change the vibe for a mission or two I recommend AKVALAB.