The sound events for Skeleton Horses galloping and stepping, "minecraft:entity.skeleton_horse.gallop" and "minecraft:entity.skeleton_horse.step", are played regardless of whether or not the Skeleton Horse is in water.
This leaves the sound events, "minecraft:entity.skeleton_horse.gallop_water" and "minecraft:entity.skeleton_horse.step_water" unused.
Iām able to confirm this issue. It was introduced in 23w12a, presumably due to the addition of combination step sounds.