What's a good use for Watersweep?

Units like Clarisse are useful with it if you don't want her getting hurt by mages while doing debuffs. I would just wait until you get a good IV 4*, and upgrade that to 5* and get a +2.

I use it for her https://imgur.com/eZxrjPB. It was worth though!

You can stuff your Soren on a very speedy ranged unit with low defenses.

Someone like Felicia / POlivia can make good use of it. With Saizo's smoke dagger (debuff) or a poison dagger, they can become more dangerous than what they used to be :)