Aside from filling the Pokedex, the next thing trainers love doing is collecting badges and medals. While some badges are easy to obtain, some are difficult. Moreover, Niantic has added some special badges if you created a specific type of Pokemon. When it comes to this, those water-types you're thinking of transferring to Professor Willow might come in very handy and here's the reason why.
Medals are basically categorized as bronze, silver, and gold. Bronze and silver are easier to gain, all you need to get the Bronze medal is 50 Pokemon while a Silver requires 100. But gaining a Gold medal requires you to catch 200 Pokemon and we're not just talking about any Pokemon but certain types of critters.
That's why some specific-type medals are difficult to acquire because some specific types of Pokemon are also difficult to find. For example, if you want to acquire the Skier Medal you need to evolve certain types of Pokemon instead of hunting them.
Basically, a Skier medal requires you to catch ice-type Pokemon, such as Dewgong and Lapras. However, these types of Pokemon are difficult to find, especially if you live in a place where there's not a lot of water biome. Moreover, the required Pokemon for that turns out to be all water-types, such as Slowpoke and Shellder.
As difficult it is to catch ice types, it is easier to catch water-types. All you need to do is catch a lot of Seeler, Shelder, or Slowpoke and evolve them. It might take 50 candies to evolve each of them but you have better chances than finding Jynx or Lapras.
So if you really want that Skier medal achievement, start collecting water types and when you have them, don't give all of them to Professor Willow yet.