The game was very neat, but a bit short. I really love the hud, it looks amazing and seems to store a lot of useful info in one compact bundle (good job!) The same two enemies got a tad repetetive, but the final boss was interesting. My main feedback is regarding the game's difficulty. I played through the whole thing, first try, no damage. It is the first area of the game, but I don't think it should be that easy. I had to play a second time, just to see what happens when you die / take damage. Another slightly annoying thing was how the enemies were bullet sponges, and took 10-20 hits to kill, although that did make the gun upgrades all the more sweet.
Overall though, it's a really solid game, and I think the game needs more enemies like the boss. There needs to be more projectiles, or some strange moving critter that actually poses a threat. Still a fun game, and simply navigating the reef was interesting.
One final thing, is the game name supposed to be 'Unknown Abyss' or 'Uknown Abyss'? I think that may be a typo?