Even with a dagger, I feel like it would be hard to fight a slaughterfish (Argonians should be able to fight underwater easily). I guess with swift swim or something like that it could be viable, obviously power attacks would drain your breath.
For that matter, lack of a breath meter seems a bigger issue.
Ever tried fighting underwater in real life? It's very hard even with a dagger. But I get what you're saying, it's annoying to have to find a shore and then wait for the slaughterfish to get close enough for you to be able to actually hit them

They should give you the ability to use your fists underwater, strangle the fish or something xD
Ways to deal with a Shark Attack
Punch it's nose
Punch it's gills.
Stab it in the nose
Stab it in the gills.
Issue with fighting Slaughterfish answered?
Also, I play underwater hockey. You'd be surprised how much damage the human body can inflict underwater. And how much precision we have with a dagger shaped object. With a dagger, it's easy. All you've got to do is stab or cut. And with a fish, all you have to do is stab it in the gills/anywhere in the soft underbelly. If it's stupid enough to bite you, you've already won because it can't hit and run faster than you can insert a knife into its body..
Likewise, to strange a fish, insert two fingers into its gills on one side. I've heard thats one way to kill a fish. You do, after all, have your fingers next to a lot more vital organs then would be comfortable.