Wow! Very cool idea!!

A few notes (since you asked) ...

- I love the small details you've added (like the price tag on the santa hat and the "gill" pattern on the hero's outfit) ... but I think you could have taken it a bit further. While the subtle details are nice, they're easy to miss. It would help if you had something a bit more prominent to add some charm. Just as an example off the top of my head - maybe the swordfish has a fishbowl helmet over his head (so he can breathe on land) but his nose has to poke out of a hole in it or something ... just a little bit of added humor to bring originality and personality to the characters.

- It's going to be difficult for me to give feedback on your colouring since I'm not familiar enough with traditional media to give proper feedback. I can only tell you what works for me when colouring my pieces digitally: I always use different temperatures for my highlights and shadows (if one is warm, the other is cool) and I always make sure to either use high-contrast or high-detail to draw the viewer's eye to the focal point of my piece.

With all of that said, this is a great idea and an impressive use of watercolour (again - something I'm not brave enough to delve into!) ... you should be very proud! =)

Best of luck to you!