I play on zedwork server. I have a small island, house and a larger created platform. The house and platform are separated by about 200 blocks,and beyond that it is several hundred blocks to any other landform.
During two seperate thunderstorms four trap skeleton horses were spawned in the water. Three of the four, in each group, were spawned beyond twenty blocks from any land. The fourth was within bobbing distance.
Total of eight skeleton trap horses spawned during two storm events.
In case it's useful there is a water monument off shore of the island, within 200 blocks.
Skeleton trap horses do not use the normal spawning algorithm. They are spawned where lightning strikes, so if lightning strikes the ocean, they can spawn there.