Ah, yes, that's why

I watched a video of a whale breaching, and heard it was to help get rid of whale lice.

OK, whales don't have hands to groom the way monkeys do in the ubiquitous monkey-grooming videos, but they could still brush up against each other and help brush crud off, couldn't they?

It turns out the lice like to hang out around barnacles on the whales; the water moves slower there.

Maybe it could be a two-fer--whales could maybe rub off barnacles too. They do against beaches and boats.

I forgot. Barnacles are kind of pointy at that end. The whales would scratch each other--not quite what the doctor ordered. Brushing might be useful prophylactically, to keep barnacles from getting ahold in the first place, but not once they get started. And most whales don't seem to be very cuddly creatures--though some dolphins swim very close together.

If sharks were more social, they might get rid of remoras fairly easily that way--sand them off each other.