Doable! Buuuut rendering hair material itself involves transparency work/material storage/etc. Right now UE4 is “just deferred” with limitations on materials and transparencies. How Epic will go about extending is a big question, because it is by no means a “one solution fits all” problem, and thus at the moment is a big area of research for many.
Whatever their solution is, it will have to consider code readability and complexity, AKA how much of a pain it is to write, understand, and modify. As well as platform cross compatibility (what shader functions does it need? How much bandwidth or whatever does it take up?) and a lot of other things. Getting it “right” for UE4 will be difficult, so I wouldn’t expect it, and thus a fur/hair solution that looks good, to be out for a while, at least from Epic themselves.
Thing is, I trust that Epic is working on a hair solution, but for water I am not too sure. I was wondering if I am better off trying to pay someone to do it or waiting half a year, since I do have a while until I will need it.