Extremely difficult to reliably row around and away from Crab King with two people, as the row animation sometimes plays, sometimes doesn't.
Also, when one player is hitting him during the health regen phase, continual melee or ranged attacks won't reliably stop him from completing regen animations.
It doesn't matter if it's the host or a client rowing, when you need to reliably row away, there's a lag between rowing and the boat moving.
Sometimes all of your previous rows suddenly catch up to you, sometimes they never worked in the first place.
I think it could be the auto targeting thinking the plank wants to be extended / mounted with Circle, rather than the oar be used with Circle.
Clear out sea stacks around Crab King.
Sail to Crab King on one boat with two people.
Start the fight.
Have one person fight and the other person handle navigation.
Experience difficulties making each row.
Fight until health regen phase.
Have one person blow dart or melee him, while the other handles claws etc.
Experience difficulties interrupting the health regen.
Try swapping roles between the host and client to see if it makes a difference.
Try locking target on Crab King for slightly more reliable results.