This is such an fun and flavorful concept, well-executed with great art. The player character concepts stand out as a particular highlight, but the new underwater mechanics also feel useful and robust. The contained adventure is also well-done, with a central concept that should play on the heartstrings of most players. My only wish that wasn't quite fulfilled- there was not much in the way of using submarine vehicles to navigate, live, and combat sea creatures. But that's really just taste. Anyone who would like to take adventures deep under the sea will find this worth their purchase.

D&D has a good ruleset for basic situations, but a lot of times, when DMs want to get into nonstandard settings and situations, they have to come up with their own solutions. That being said, if you want to run a game underwater, this is an incredible supplement, incorporating ideas like buoyancy, pressure, and the different depth zones underwater to craft a ruleset that takes excellent inspiration from real life, but ends up being fantastical enough to work in 5e. On top of the rules, there's a bunch of wonderful player options (including the carcinization of D&D), some delightful monsters, and an excellent, inventive mini-campaign that I would be incredibly excited to play or run with a table of players. Check it out!