Hi I think you have 2 interesting characters there. I would agree with what others have said and do more thumbnail ideas.
If I was doing more thumbs I would look at avoiding splitting the page in half with your horizon line.
Also with perspective you would see less detail in the distance so the ripples would not be as prominent or would gradually fade as you go further back to the horizon.
On the concept it doesn't really tell much about the story, but that could be a preference, however the background is quiet bare so a bit more character there might help.
I know this is a mock up, so I'm sorry if i'm pointing out stuff you know or will look at. The contact shadow under the character on the left would help cement that character in their world.
I would have a bigger title, maybe no black banner, you could mess around with this in your thumbnails. On the fonts i'm not sure the bottom one works well here. Also the 'Jazz & Rabbot' text, maybe you could play around with the layout size etc. maybe have a look at other titles and see how they are laid out.
Hope that helps, the characters are cool, can't wait to see more progress.