I like how it's looking...
What you have so far looks great. Like SpiritCrusher, I think that the underwater crawlas would be somewhat silly and if you can't figure out what enemies to put in some sections, I say none at all, especially it it's a small room. Clever placement of mines can create a unique challenge, in my opinion, and the same could go for Jet Jaws; sometimes, it's not the enemies themselves that create the challenge, but the placement. Of course, the general consensus will always be that crawlas should generally be omitted from a level that supposed to be mid difficulty.
The visuals and scenery are definitely looking nice and I like your methodology about the transition between the underwater area and the mechanical area (and vise-versa).
Hope I helped.