(At time of writing I haven't played the DLC yet and there are a couple of rare fish I'm missing, but I've completed the main quests)
Well, it's a fishing game, there isn't a whole lot to say about this. You sail around a cell shaded ocean and get some fish. It's not a difficult or challenging game - the fishing itself is done through QTEs, which are varied and fast enough to not get annoying, and the trading economy isn't very demanding. You have to do some Resident-Evil-style inventory tetris to get the most out of your trips, but it's easy to keep up with your monetary needs for ship upgrades. But the ease and simplicity works in its favor, it's enough to keep you engaged and not too demanding or time consuming to get frustrating. The real draw here is to explore the ocean, find the different fish and figure out how to catch them, and the simple gameplay makes for a satisfying loop. From my experience, you never really feel stuck, there's always something new to find or a short term goal to work towards.

The main story is alright, the atmosphere is really good, so it overall comes together to a very relaxing experience. This might sound odd because of the horror game elements - the game has a panic meter and there is something evil lurking in the deep that twists the fish into grotesque mutations. However, high panic isn't hard to deal with, and the mutated fish have cool and creative designs, so collecting them is more reward then a threat.

Overall it's a relaxing and atmospheric experience that managed to consistently have enough discoveries and goals in store to avoid becoming boring.