There was a lot of depth in here! The ring out death mechanic is unique for the genre and there's plenty of enemy variety, which is pretty awesome! I also loved the drifting lights over the rooms-- they added an extra layer of atmosphere to the game to really sell the feeling of being at the bottom of the ocean. The game is also surprisingly unforgiving, which I enjoyed because it's the fair kind of unforgiving. The enemies and most of the items feel really well balanced!
Two factors that frustrated me were the wonky aim controls of the bombardier shell and the existence of the shell-less form. These may be skill problems, but they still took me out of the experience. There isn't any indication that the bombardier shell is tied to arrow keys/directional movement, so coupled with the fragility and lack of movement options that it has it felt very frustrating to play. The range that was meant to make up for those options being absent felt too unreliable since I didn't know whether it would actually go where I wanted or off in a diagonal direction because of how I was moving. The shell-less form (unless I just couldn't figure out how to use it) has no way of interacting with enemies, so being knocked into it is basically a death sentence anyway. It feels strange that you don't just end your run immediately after getting your shell broken because there's no functional difference.
That being said, Shell Collector keeps me coming back anyways. It's fun and easily replayable for sure. I wasn't sure what to expect coming in, but now that I'm here I'll definitely stay beyond just a few runs.