It's a really old game that came on one of those systems that used cassette tapes and took forever to load. You could select different planets to go to (each planet was one level) , I remember there was what I think was a "city" level and an underwater level, and the underwater level had a huge crab or something that traveled along a fixed path. The objective was to collect blue orbs of some sort and then probably return to your starting position (marked by a white box that might have been an elevator door or something). You couldn't really attack anything, you just had to avoid the bad guys (of which I think there weren't that many, most obstacles were environmental). 2D Side-scrolling.
Wow, I just remembered something else. The music of the game was, or was very similar, to this thing here:
http://www.youtube.com/watch?v=CQdgJYhsrc0
(Blue Danube Waltz by Strauss)
I have made a mock-screenshot in paint of something from the underwater level. A formation of spikes you can't touch, there's a blue orb inside it and the white thing is your character. You could 'swim' in the water by jumping repeatedly. (please note that my memory of the game has been distorted by time):