Good job :) I like the extra enemies as they seem pretty easy to implement.
The level itself, although at first glance seems like a lot, I would say it actually falls a bit short. Consider at least adding some more complexity to the final room, as it is, it's just a few rather easy jumps.
For the underwater design - while definitely an interesting take, the 2DGame Kit doesn't really support this art wise, so the question is how hard would it be to supplement all these elements, so it does work visually. This also begs the question of - why can't the player swim?
For the "main character" - is this the player character? You state that the new enemies don't need custom sprites, but you don't state this for the "Sunny" character. Also why is the Sun underwater?
For the design document itself. There is way too much text. Emit any filler text such as "If you find better settings during testing, feel free to change them" - this goes without saying for any dd. Also read it after yourself at least once, there is way too many typos (the text I just mentioned is a good example) and misspellings. I would argue that the technical sections should be omitted completely or very limited. You are in the role of a designer now not the programmer. Though I can understand the need for these suggestions, keep them to a minimum.
While I appreciate the use of a legend, it's very messy. Try to line up the pictures and description properly.
To give an example to some of my points - The Level design section feels rather obsolete - what is the meaning of the white box? The description next to it doesn't really say anything not already communicated by the level map and the information for the lever is then later repeated in the legend in the "Lever" part. All while not answering a question of - can the player see the jellies before they appear? AKA what's the camera size?