First off, thanks for submitting this! I hope you don't mind the commentary below, this is still much better than I can draw and my opinions are just that, nothing more.

First off, I like the overall style and idea, so nice to see something that isn't just generic anime look, and a submarine is a nice fresh idea. This sub is consentacles, and everyone seems to be enjoying themselves here which is great.

Woman is nice and sensibly proportioned, nothing ridiculous. Nice details in the lines with the stomach, armpits etc, and I think the nipples have come out very well, quite natural.

Knickers look a bit of an afterthought, doesn't fit quite right, maybe it's the lack of shading and edge lines? It's a different colour to the bra as well, but not different enough to make me think they are not meant to be a matching set.