Only way I've found so far is to go back out of the dungeon (very carefully, healing Arche as I go since Sana isn't awake enough to do that for me), get over to an inn and rest (you can use healing items on the KOed party member once you're out of the dungeon, but the inn is right there and a cheaper way to heal). The incremental progress, XP and cash I make each dive makes it worth coming back outside rather than just killing the game and restarting (with the shield and enough spellcasting I can usually get Sana to the next water draining switch along before she runs out of MP).
Hopefully as I get better I will have to do this less (I still have to make a conscious effort not to just hammer the attack key in the heat of a fight!)