It can be a pain in singles, because despite its high offences, it has 45 base speed, which let's it's butt kicked by most other water types. From what I can gather, it absolutely needs cleric support to stay at full health to stay in School Form, which is kinda hard due to its already mentioned slow speed. Otherwise, you're better off with pretty much any other Water-type.
If you are planning for the VGC or doubles, it and Oranguru are going to be an amazing team. Oranguru sets up Trick Room, then spam Instruct Surf and Earthquake. Ribombee will also be a good partner, since it can Speed Swap its blistering 124 base speed onto Wishiwashi, keep Wishiwashi healthy with Pollen Puff, and temporarily cripple an opponent by Speed Swapping Wishiwashi's speed onto them.
If you just want it as an in game pokemon... it's your choice :-)