Horses kick you off when you pass through a waterfall, for example a waterfall like the one shown in the picture below.
MC-20993
Horses kick you off in waterfalls.
Resolved
Duplicate
0
08Juan80
(Unassigned)
Unconfirmed
Minecraft 1.6.1
Retrieved
But this relates to waterfalls, water moving, not to not moving water, for example a lake.
So this is a bit different than the one that relates to horses kicking you off in water deeper than one block. Waterfalls are a bit different by the way.