Resolution: Works As Intended
Affects Version/s: Minecraft 15w51a, Minecraft 15w51b, Minecraft 1.9
Fix Version/s: None
Prior to week 51, frost walker boots caused even ridden horses to freeze water, so you could ride a horse over the ocean.
MC-90585 changed things to fix a bug. It is unclear if the horse-frost-walking was originally an intentional feature which the other fix broke, or if it riding horses across oceans was never intended to be a feature. (I liked it, but it's up to the devs.)
Another option is to also resolve
MC-93570 by having "Frost Walker I" not cause horses to frost, but "Frost Walker II" would causes horses to frost. (Right now there is little difference between different levels of Frost Walker, that could be an interesting distinction maybe.)