I noticed the sub has a health bar but is currently unused, are you planning sub attacks or sub support in missions?

in the text encounters that happen on the map some of the outcomes will cause the sub to be damaged or repaired.

In Build 37 we will have battles taking place between characters on top of the subs. We don't have that having any effect on the sub itself right now. It's possible that could change in a future build but it's not part of the plan at the moment.