I liked this game and played it a few times before posting this feedback. I think the concept is great, the theme is really well done, the controls are intuitive and the "arrow of play" (where to go, what to do, etc.) is clear.
What I think you can improve on: it's hard to tell what pickups/gems do. Often, I end up risking a bunch of fuel only to get a hull upgrade when I have full health, or some such. It would be really beneficial if I could tell what pickups are when I can see them (I didn't notice if there was a consistent pattern, eg. yellow = fuel).
Also, the fuel management part is really hard. It's really easy to swim down the wrong (empty) section of a map and end up blowing out your fuel budget. Not sure how you can improve this though.