i like the concept. my only suggestion is that you should probably define the subjects a little better!

Very nice idea, I like it a lot.
Some more time could be put into the background and general detail, but its a top notch job so far.

Kudos, on coloring the 'frame' (people always forget that). You have plenty of time to add details and more textures. It will make the whole scene much better, interesting and appealing. For example he could be sending a message in a bottle out. Or a mermaid popping up if you can animate. A thougth bubble...think about it. Your base idea is all there and working just fine. Polishing will help a lot. Remember its a challenge :)