Good for a prototype. I like that it's displayed as if it was a TV screen, the graphics are also all right. Shooting and damaging enemies work perfectly.
Nevertheless, what's the goal of the game? Currently, you can't lose (no matter how many enemies slip past you) and you also can't win (i.e. by defeating several waves of enemies).
If it's supposed to be an endless game, you ought to display the current score and the game should get gradually harder.
If it's supposed to be a game with different levels (based on the information about "Location"), you should show "remaining: 20" or "Wave: 3/7" or something like that, and then move to the next location.
Currently, the enemies also get stuck at the edges of the screen if you let a lot of them pass through you.
In both cases, I think the enemies should take fewer hits to kill and there should be more of them. And different kinds (e.g. tougher ones requiring more hints, enemies which switch places when shot etc.).
As for the main character, he doesn't appear as if he really is in the sewer – he casts no shadow on the sewer walls and moves along a circle even though the sewer is a 16-sided prism. Another game mechanics might be to make the water slow you down slightly, so you'd have to decide whether to go through it or rather cycle around. And the enemies also always overlap you even though they should be in front of you at some point, which makes you look flat.
There are also no sounds nor music.
Overall, it's a very good prototype – but as a full game, it's incomplete.