Remove automatic dismounting of mobs in Water (or make it toggled)

The automatic dismounting of mobs in water was added with horses, where you would automatically be kicked off the mount if you were submerged in water, though this does not apply to Minecarts, Snowballs, or Boats.

The logic behind this feature was that it would encourage you to use boats for water travel instead of horses by interfering with their ability to navigate it, but now that horses have been in the game for a long time, it has proven they don't really need that nerf, as they are underpowered as is. In addition, the new world generation has made rivers much larger to go across, making horses an even less desirable choice for any travel, as you are bound to hit a river/ocean at some point. This is also inconsistent feature, as it doesn't apply to minecarts, snowballs or boats, as said before. From a mapmaker side of things, the automatic dismount of mobs riding each other make custom mobs a difficult to make as they come undone the second they touch water.

Instead, this should be made an NBT tag (DismountInWater) or an accessibility feature, and replace it with a 60% movement penalty when horses are in water. This will keep boats preferable to horses in water, but allow water to still be traversible with a horse for short distances. In addition, this would allow mapmakers to make mob jockeys that don't come undone the second they touch water, or at least control it.