It sure won't be a trout
The obvious con to hair in water is more drag, so more energy required, so more food needed. That is an evolutionary disadvantage, because more food means more going out there to find it, and means the same supply feeds less individuals. One outcome is the hairy fishes are gradually phased out of evolution for lack of surviving, and that's no fun. Another outcome is they evolve in a different direction.
To offset the disadvantage, your fish need to move as little as possible. That makes it an ambush predator, capable to pounce at incredible speed unto unsuspecting prey, but overall below par as a swimmer. It doesn't need to swim fast and far anyways, and it won't try to eat anything that can escape the initial attack. This is already a thing, so it's not that outlandish to imagine. If you want to increase the weirdness factor, you could imagine a fish that hunts with its tongue like a chameleon. Although I don't know how well that'd work underwater, that would be pretty weird.
It would use its hair to camouflage in the environment, to surprise smaller fry and to hide from bigger fishes who would outpace it. It would of course help to live in a hairy environment, for instance if you had hairy coral or hairy seaweed, or at least if those pass as hairy in the eyes of a fish. It would possibly develop a slower metabolism to decrease the energy input and the amount of time it needs to feed, though that means doubling down on the "moving as little as possible" lifestyle. They might prefer warm waters, that much less energy to spend in heating.