This was a solid level. I liked how this level made use of three different ideas, that being the rotating platforms in section one, the jumping Cheep Cheeps in section two and the dolphins in section four, but you could've tried to connect them more, because I felt like I was playing three different levels consecutively. The only real connection these elements have is the very last bit of section four, where the player has to jump from the platforms onto the dolphins.
(The segment I'm talking about, at the very end of section 4)
In my opinion, this was the highlight of the level, as I had to jump from the platforms onto the dolphins in order to avoid the enemies and then manipulate the dolphin's jumping height so I can reach the following platforms. Still, the lack of jumping Cheep Cheeps in this part is bothering me. You could've placed them below some of the rotating platforms or between the dolphins and platforms in order to increase the overall challenge.
Furthermore, I have three relatively minor additional issues
with this level. For one thing,
This pit is almost inescapable. I played as big Mario, fell off the first platform and barely managed to reach the left "?" without dying. Pretty sure that it is entirely inescapable if you play a character with worse jumps or if you fall off one of the later platforms. Just make this a bottomless pit and it wouldn't bother me at all.
Same can be said about this segment. Just remove the water and turn it into a pit.
Regarding the visuals and sound, this level certainly looks and sounds like something from SMW. There's not much else to say about them, really.
Like I said at the beginning, this level is solid. Neither amazing nor bad. It has interesting ideas, but doesn't use them to their fullest potential. If you want a number from me, have a 6.