Don't know if anyone has noticed but in one of the levels, if you finish it, even though you're not in the water, I'm guessing sonicworlds uses weird collisions however you start drowning and die before the score is finished. However, I found a bubble sheild in the level somewere and that stopped it but the warning sound still played, the "DingDing!" sound.
I really enojyed the graphics however with what has been said above, I have the say with some of the graphics is extreamly blured. This ruined the titlescreen for me and It looks like you mega resized it to fit a 320x240 screen.
I really liked the music, seemed to fit well, Did you make them yourself?
The levels were quite good, you had an element of variety which is a excellent feature, however their are some things which need fixing, small little bugs and placment of objects. Sometime i can't get past a spike without hitting it.
Overall, I enjoyed this demo. It definatly has potential but it just needs a few fixes here and their, including presentation as i stated above. The Titlescreen being an example of this issue.