This is a few vehicle upgrades in one, my idea stems from the biomod system.
1): similar to below zero, maybe an upgrade that increases speed, could be a boost or it could be an enhancement to engines increasing/improving them bringing a better top speed. You could use this with or without the engine efficiency, without it would use more power, with it would balance(still using slightly more power than without the speed upgrade). That way its not too overpowered, still feels rewarding without being overkill.
2): maybe a similar system to biomods where you can upgrade 2 additional slots, 1 active that you manually control like a shock defence upgrade? Similar to the Below Zero. Or a passive one like a reserve powercell pack(similar to the base power storage), once your submarine(s) are fully charged and are still inside the base(or using the charger upgrade), the storage will then charge up giving you more power, maybe make it half of a powercell so its not too much but still feels like it is worth it?.
3): some ideas for passive/active from 2 could be:
1): (active),a dash like ability where you can boost either forwards, backwards or sidewards briefly to avoid predators similar to the biomod ability(which is awesome and handy)
2): (passive), this is a more hear me out, an upgrade that consumes power to self repair submarines you would use a repair tool as part of the crafting. The exact % of repair i am not entirely sure on, yous can decide that. Doesn’t have to be a full repair either, maybe it kicks in when you hit below 25 health and consumes 1 power per 1 health up to half or 3/4 health?.
These are ideas to consider for after the starter area, alongside the story updates in the future. So that they aren’t too OP early on, can be mid to late game depending on how powerful you think they are, just upgrades to consider because the biomod system is awesome. Maybe a similar Mk system for each one, the further you progress the better they are, so for health maybe Mk 1 gets you 25 heal(mid game), Mk2 gets you 50(late game).