When a trapped skeleton is activated underwater, the skeletons on horseback seem to be able to move in all cardinal directions at a super speed which doesn't seem natural, including "coasting" for long distances in horizontal directions after moving off the edge of a block.

How to reproduce

1. Go to an ocean biome and go down to or near the ocean floor
2. Add some buffs for your convenience:

/effect give @p minecraft:night_vision 9999 1 true
{{ /effect give @p minecraft:water_breathing 9999 1 true}}
{{ /effect give @p minecraft:resistance 9999 100 true}}

1. /gamemode survival
2. /summon skeleton_horse ~ ~ ~ {SkeletonTrap:1}
3. Watch the behaviour of the skeleton horses underwater

Notice how they move extremely quickly, jump up blocks and then ride off them in a horizontal fashion with no immediate downward momentum.