I had a *lot* of trouble getting anywhere in this game, between the short oxygen timer, the inaccurate sonar, and the small number of hits you can take. And all the upgrades seemed so far away considering how limited your oxygen supply is at first, so finding even *one* gold nugget was difficult (let alone getting back before your oxygen ran out and/or you smacked into too many walls trying to hurry). And since you had to pay to repair damage, that made it even harder to get the other upgrades (which required ten, fifteen, or even twenty nuggets).
I like the concept, it's just very difficult to actually play without blowing up repeatedly. :D