Yeah, the big thing to notice about ponies is how much some of the lines actually curve. In particular the ones that define the front part of each leg are challenging to get right, because they have to be somewhat concave to give them the right look without seeming fat, but too much curve and they seem abnormal. I find I constantly have to go back and tweak lines I've drawn to shallow or to straight during my rough layout.
Hmm, I would go with a blue-black for the wetsuit, and similar colors for the diving gear. The patches you could probably go wild with, but I'd imagine the cutie mark one should look a bit like her normal color scheme, and the Spec Ops one would probably use the sort of color palette they do in real life - mostly dark browns, greens, blues and whites, but some primary colors (especially yellow and red) for accents. Besides that, it's mostly just AJ's body, so that should be straightforward.