This is a great soundtrack. Dynamic music that changes based on the environment is always very cool. I also like the multiple blend of themes, such as underwater and mechanical, that still keep well thematically. There is a good variety of feelings - safeness, foreboding, mystery, intensity, etc. I think it is very cohesive.
Someone mentioned "sounding midi", I don't think that is a bad thing. Unless if you are going for realism, it can be a great tool to use midi sounding instruments to achieve a game feel.
Overall, this was a nice listen, Great work!