Blocks of water render their inner texture planes in cases where they would be expected to be visible. This is actually inconsistent with other translucent blocks such as ice, where they are never rendered and are not even defined on the inside.

A resource pack that fixes this for all full translucent cubes is attached (purely transparent blocks such as glass, as well as some non-full translucent blocks such as stained glass panes, are unaffected by this resource pack). It is worth noting that this fix exposes some further bugs with the vanilla game though, such as major Z-fighting when a fixed block is connected to a block such as a wall, many blocks gaining a dark inner face when a solid block exists opposite it, and the inventory items appearing to be missing some faces.