I have not seen a level any higher than that unless it were the Death Omen as Rackat mentioned.. dont remember now. But have seen the level 45 Jugg and sank her before myself
The higher the level ship, the more cannon's it takes ta bring'em down generally (more damage they take). This adds up for more cannon/sailing rep during battle. Many of the middle level ships will give this early on in your pirates ventures. As you progress however you will find the need to seek out those bigger ships to maintain good rep points (and plunder) fer yer troubles.