Just beat the final boss. Pretty exhilarating going up against that huge battlesub with the manatee. The game could definitely use more missions. I'm having fun with time trials, but I had the most fun just trying to flat out beat the boss, unconcerned with time.
One of my problems with time trials is that I feel I have to get the decals in order to have a shot to beat anything past the bronze medals. And getting the decals is a bit of a grind. I wish they activated after you got two of a kind, rather than seven of some.
Overall, though, it's a lot of fun. I don't know how I'll fare against the boss when I have to use the really big sub. I can imagine taking a lot of damage from successive hits. Looking forward to doing that, and then Expert mode.
Also, I think it'd be interesting if destroying enemy subs gave you more time on the clock. As is, the time-based gameplay encourages you to race past and avoid combat, but it might be interesting if it was a risk/reward situation where you could get a decent chunk of time on the clock if you stuck around and destroyed a ship quickly.