There is nothing I would change on this remake, I love it!
It's loosely based on a Dick Smith Wizzard/Creativision title called Air Sea Attack, but that game was a lot simpler.
It's a shooter basically. You steer your submarine left and right on a scrolling underwater vista, shooting a wide variety of enemies which are out to get you. Each enemy is pretty cool and has its own unique method of dispatching you. I particularly like the scuba divers that try and plant a limpet mine on your sub, which can only be dislodged by getting a little too close to an explosion. And, there's also a decent boss battle at the end.
This remake is now complete, and the file can be downloaded from here