Underdeep potion healing in combat is usually a mistake: a potion restores 50% of your total HP, but drinking one inside a fight consumes the turn, and the damage you take in that same turn can match or exceed the amount you just healed. The working answer is to drink between fights with the E marker instead.
Underdeep potion healing in combat: what a potion restores
A potion restores 50% of your total HP. You start with one potion and can carry more; the character panel shows it as Potion 1/1, meaning one carried against a carrying capacity of one, and a potion-capacity skill raises that ceiling.
Drinking inside a fight usually backfires
Healing from inside the fight consumes the turn. Because the enemy still acts in that turn, the damage you take can match or exceed the heal, and you can end the fight worse off than you started it. The trade only makes sense when your health pool is so large that half of it is still more than the worst hit you take.
Three things the game does not do yet: drinking a potion outside combat, healing that does not consume the turn, and a potion marker that reacts to your current HP rather than firing on a fixed tile.
The E marker is the real fix
The practical solution is the E marker (Use Potion) placed on the tile you leave after a boss fight. The character drinks between fights, where nothing can hit it, and keeps going - standard practice on the first dungeon's mimic route. Healing from a marker avoids the turn cost, but the marker fires on the tile rather than on your health, so it does not consider how hurt you actually are. Placing the marker on the tile you leave after the fight is what makes it work, and the marker system is covered in the route settings guide.
Potion capacity and the skills to skip
Potion capacity is worth a skill slot when your damage or health cannot kill a boss before it kills you. Skills that make the character drink potions automatically are not worth buying. The full buy and skip list is in the skills and Orbs guide.