Fun game, cool premise! I enjoyed playing this game, the powerups were a lot of fun and added good variety. The health drops were a little over abundant, but also made the game forgiving, which is good for a jam game. I enjoy a bit of challenge to my schmups, but still like this one.
The 3d models were very well done, but I thought the water texture could have used some improvement, especially since you spend so much time looking at it. A bit larger UV scaling would have made the waves more realistic, and the bottom of the tub seen through the translucent water would have really sold the setting.
The controls were decent, but a bit rough around the edges. Laser firing didn't consistently start on mouse down or stop on mouse up. Also, the movement speed was great, but a bit of inertia to the directional movement would have made handling the duck more fluid and perhaps felt more like floating on water.
I played this game through 30 waves, and had a really fun time evading and shooting all the ships. On the later levels, I found I could corral all the ships into a pile and annihilate them all with 1 bomb, which was optimal, but not as much fun. Post-level-30, I decided to die, but found that all the enemies tunneled beneath the water trying to get to me and I couldn't easily die. This may have started on an earlier wave, which would have contributed to the ease with which I could corral all the ships.
Finally, in the later levels, there are so many ships that bombing a good chunk of them frame-lags the game, and even my GTX970 was having a rough time with framerate. I'm not familiar with Unity (UE4 here), but I expect it wouldn't be too hard to batch ship removal over several frames if too many of them die at once. The visual impact to the player would likely be negligible, and it would remove the noticeable hang when a bomb kills large numbers of ships.
Great game, thanks for the fun! Keep on developing!
Here's a quick video I made of the OP bomb strategy I found, as well as the weird late-game ship diving behavior:
https://www.youtube.com/watch?v=08ZVyCbK5sE