i love the boat too. Controlling a boat mixed with the shooter bits is quite compelling. Boat customization/upgrades would be cool and make it feel like a base.
Water physics are pretty awesome, things like blasting water with a shotgun having an effect
I missed the shotgun and speargun the first time through. I also never figured out how to use mutagen.
Shield guys will charge off a bridge to try to get to you, and I think the spear throwers can keep throwing while underwater, which was frustrating and I couldn't tell where they were.
All settings get reset between levels, although this might be a problem with my sandbox
I feel like the game needs more health packs.
The lighting/ambience needs work, and at night, I found it kind of hard to see.