As others have said, the overall level of the tracks is quite low - definitely worth boosting them up.
That aside, these are pretty good! Nice arrangements and the orchestral elements are well placed and articulated. The use of the bubble SFX to sell the underwater theme is cute, and they're sitting just a little back in the mix to prevent them from being too overpowering.
My favorite of the bunch is Puzzles and Mazes. You've got a knack for catchy and well thought-out progressions. Kudos for an almost minimalistic approach at points - I noticed that a couple of the tracks allow ample time to breathe and not become to busy or 'full' with instrumentation for the sake of it, and I think this track is a great example of that.
All in all, great work. The volume thing is a non-issue in the grand scheme of things - it doesn't detract from the quality and thought put into the arrangements.