Merfolk have special oils in their hair that makes it waterproof! The purpose of the oils is to reduce heat loss through the head, and as a lovely bonus it makes merfolk look immediately presentable after getting out of the water. Straight from the beach to a job interview lol. If the oils are washed away or harmed, the hairs absorb water instead (which used to be a big deal like a thousand years ago when hypothermia was a very real and concrete threat, but in the modern day it's not really an issue).
Merfolk also have waterproof clothing, it's just a lot more comfortable that way. Most of the time clothing is made waterproof through spells, a method that merfolk pioneered and mastered over the centuries.













