Great game with a few diffrent strategies to try.
1: Ship weapons sometimes vanish and never come back. I've been well into the game and then lost because the ship was defenseless.
2: The bomber units that sit at the bottom of the screen and spam mines. These get a little annoying as they deal tons of damage over time. Maybe shorten the firing rate or slow the speed of the mines rising?
3: The other Mermen seem to never fire their missiles or vulcans?