Thanks for the feedback!
Visual feedback when taking damage is on the to-do list.
Each ship has starting HP and max HP. Certain crew members increase your start HP, so it is possible to begin a map with more than your max - these are the blue blips on your HP bar (similar to LifeBlood in Hollow Knight), and they are not recoverable if you take damage.
Heart pickups do refill red HP blips though.