The Phantom is the lowest level of the Skeleton ships. So if it was a Phantom it was probably in the 16/17 range.
It is possible for him to sink it if he knows how to sail and stay in front of the ship so that it doesn't damage him. If he's level 1 in sailing though, it probably took him a long time to sink it, if it had full health.
Another thing that could have happened is that the Phantom was already low in health, and he just need to hit it a few times to sink it.
