I finished playing through the level. The opening is a big no-no: don't place an enemy right where the level begins. While it is harder, it's because of enemy spam and few powerups in relation to that. In some situations, particularly where you have a Clappin' Chuck under a cloud or blocking a corridor, or that one sitting on the muncher or between those pools of water, those are thoughtful ways of using enemies to their maximum effectiveness because of the way they interact with their environment. Back-to-back sliding Koopas coming down the pipe you're trying to scale or a train of Chucks running after you in tighter areas are not as engaging to deal with, and are more frustrating. I am surprised, given the name sake, that Pitchin' and Puntin' Chucks aren't used, because those are enemies that can be used in some of the most compelling obstacles: there are places Pitchin' Chuck will fit right into your design provided there is a Koopa nearby, and parts can be redesigned to make Puntin' Chuck effective with or without being close to Mario. (Just make sure he can be seen.)
There is an issue in the second room that can be fixed with some Map16 magic: where you have an escalator going this way / you should make the top left part of the block next to the top act like 100 so Mario doesn't stumble, and the one at the end can safely act like what it is to prevent passage through it. The way it is now, you have to go to the bottom of the escalator and jump over it or you end up jumping into the spikes. It's also very
hard to jump out of the water without going right into the spikes. I am not so sure the fish parts suit the level since it doesn't fit the concept and you can swim right under them, and these segments could be redesigned around those more advanced Chucks.
The first room looks good - though SMW comes with those palettes, it's tasteful. The second room, even though Vanilla Fortress used these foreground colors, it was pretty gross then too. I'd personally try the brown, grey, or green if you're not experimenting with custom palettes.
The level length is perfect in my opinion for this kind of level.
Just look above you...
If it's something that can be stopped, then just try to stop it!