Is there more to do than getting the handful of gold bars?
I noticed, when you're floating high enough above the water you can break through the barrier to find an island, but beyond that the movement speeds drops down significantly.
What's missing a bit is that the angle of the diver needs to correspond on whether you climb or sink, and diving "backwards" needs the diver to turn around...
Good thing that the oxygen in the tank is not consumed because the tank has already been depleted a lot and you can't get into the boat yet and exchange the tank for a fresh one. :D
Lovely graphics!