It's a nice level you have here, but I've got a few suggestions to improve it:
In the beginning of the level you can swim up the left side and can go over the level/get stuck.
When you get the fire flower (or are big and try to get it), you can't get past these jellyfish without getting hurt.
This pipe transition is unnecessary.
Your level could also use some more seaweed in the background, to liven things up a little. And in the "maze" after the midpoint, you might want to place some more fish, lava lotus, jellyfish or roto discs, to make it a bit more interesting.
Your level also uses the smb3 roulette exit, but it needs to be the smb3 star exit.
Hope this helps.