The weird thing about Crab King is for all his magic and claws it's really hard for him to actually kill you if you play your gems right. This strategy assumes you're using the easiest gem setup: 1 Pearl's Pearl + 8 Purple Gems (Level 3 in all stats, Level 11 in Number of Geysers)
Spring is the best season for him. Assuming you allow switching characters, Wolfgang using Volt Goat Chaud-Froid after a Spring rain can easily dispatch Crab King in less than a day. I start the fight with about six Dark Swords, but with this much damage you only need about two to three.
Ice Staves are easily the way to go for Crab King in my opinion. When he starts casting his geyser spell you can easily put him on ice and use the time to heal. Four Ice Staves should be enough as any character who deals regular damage.
The freeze attack is the only one that can actually kill you outright with its temperature-dropping aura. Wearing a Beefalo Hat or a Puffy/Hibearnation Vest will easily keep you from freezing. Not like you need armor anyway. You can simply run to the opposite edge of your boat to avoid being frozen as he casts the spell.
Imposing Claws will crunch up your boat if left too long, but with the damage setup mentioned before you can take them out in one hit. Keep a stack of boards, boat patches or trusty tape on you for repairs while Crab King is frozen. Also make sure your anchor is in the center of the boat, nothing worse than a claw smashing it off and creating a large leak.
As he starts healing, you usually have freshly summoned Imposing Claws latching on right as he starts repairing his crown. A single weather pain shot can block most of his rocks from reaching his head, but your damage output should work to easily cancel any that do make it through.
The Night Hands of the sea, Wavey Jones will spawn at low sanity and mess with your boat. Keep some form of sanity restoration on you while you fight Crab King and heal your sanity while he's frozen. At his worst Wavey Jones will tear up boat patches and pull your boat outright, which is an ill-afforded inconvenience.
Overall Crab King is just a dance of making sure a geyser attack doesn't get through, not getting frozen, and keeping your boat health high. If he does get you it's not a big deal, a bit of a drag to get back there but the worst of it is some max HP loss. Oh, and watch out for sharks.