I didn't spend enough time to go fully in-depth with this demo but I do like what's in here so far. I understand this is only a gameplay demo so far, and in that sense, I liked the general gameplay loop, map design, and weapon/enemy variety (however confusing that might be due to placeholder sprites).
If anything, I would appreciate some tweaks to the movement to make it feel better - not necessarily more responsive, since the setting is underwater - maybe this is more of a visuals issue? What I'm saying is that while the movement itself doesn't feel bad to me right now, something more is needed in order to sell it and make it feel good to move around in this fashion. Later on however I would expect some more mobility skills or such (though I didn't get super far in so I might've missed them).
There was also a bug I found in one room, where three or so elevators would all control themselves & the others at once with each's respective switches. Not sure if intended, but if so, it was not communicated at all.
Keep it up!