Review time!
I'm largely on the same page as Zipper when it comes to this level, so I'll try to not repeat his words too much in this review.
This is an interesting little level with tons and tons of areas to explore, kept on a scale small enough to stay interesting and not bore the player. Usually when I see levels which try to go for tons of secrets and split paths, they end them in a dead end, forcing the player to turn around or take a return pipe. This level doesn't do that. You cleverly arranged the geometry in a way where the player always has a ton of choices on where to go next, and the music you chose (while a bit TOO adventurous for my taste) supports this theme of exploring the island and choosing your own path.
Every secret area was stocked with rewards including powerups, dragon coins, 1-ups and even a secret star! Saying that this level has 6 dragon coins is probably the best way to describe just how much stuff you can find in here.
While I had a great time going from one side-area to the next and looking for the treasure of the island, there are two (or three (two of them kinda merge into one, really)) things which bothered me a bit about this level. First off, something Zipper already mentioned: The precise movement required underwater. I think this issue is kind of a result of the small scale you built your level on, and while I didn't have too much of a problem with it, I sometimes did decide to just take a hit because I knew the level would give me another powerup soon enough. What I suggest is taking away some of the enemies near small passages, or if there are multiple, taking one away. Placing enemies in an underwater area is difficult, and it's better to not try to block the player off with them most of the time. They should usually serve as an avoidable obstacle to manipulate the player's path through the level (at least that's my opinion, I'm not a fish scientist).
My second nitpick is that the second half is really lacking after the huge and expansive first one. It's just a big letdown that there's really nothing to explore and I think it could've just been an additional three screens in the first section. I might actually be a bit overly salty about this because I accidentally went into the pipe to the second half, thinking it's another secret area, and finding out that I can't go back to explore more of the first half. The lack of substance in the second half also creates the illusion that the midpoint is heavily lopsided.
Overall, this level was a lot of fun to play until the midpoint, and I suggest looking into building the 2nd half's features into the first one and moving the midpoint to approximately the middle of the first section. The sudden change of pace ripped me out of the experience and just really doesn't compliment what this level is going for. Anyways, I had a lot of fun.
7.3/10
Before I go, I took some gifs during my plathrough of very minor stuff which isn't really an issue but which I'd like you to see:
http://i.imgur.com/dvYbQum.webm
There's something going on with that torpedo ted which you might wanna check out.
http://i.imgur.com/dkuuG9s.webm
Double-check every cheep's effect. I've also seen that the red cheeps look weird when dying and this might also be true for the large ones.
http://i.imgur.com/mISSc6n.webm
Neat little shortcut I found. While it's unusual and probably unintended, it's definitely a plus to me and something which I really don't see enough.
. The secret star was pretty easy to find as well, while providing its own challenge. One problem I had is that the water areas could occasionally be TOO slow. Consider the Torpedo Teds in the underwater section. They take an eon and a half to get through the screen, since water slows them down. Same goes for some of the big fish, they require you to move VERY precisely in order to not get hit, and while this isn't hard, it DOES consume a lot of time in the process. Perhaps a way to speed up the fish and expand the area they travel would help? It'd be less of a waiting game and more of a timing game.