People are finding all sorts of delightful ways to break Zelda: Breath of the Wild, sometimes within the confines of the physics engine, and sometimes, in all new ways that Nintendo ever even thought of. This instance probably belongs in the latter category, veering into firm "this is a glitch" territory.
Twitter user Swiffy22 found that by getting hit (in this case, from a Stone Talus) and falling backwards into a body a water, that they could walk along the ground. Breath of the Wild doesn't feature any type of diving, even with Zora gear, so this is clearly breaking the game.
Still, it looks pretty harmless, especially given that you can fast travel at any point, even in combat (and the auto-save feature pops periodically even on the world map) -- a system that was probably implemented to avoid situations like this.