Considering this is a game where cutlasses look like rapiers (or some other longer straighter sword), daggers are a little longer then Roman short swords, and flintlock pistols perform and sound like semi-automatics when you load them, I'm surprised at how accurately the ships are done in POTCO. Their relative performance is very much like their historical counterparts. Also the ships are all types that could be found in the Caribbean in the time period. The sloops look like historical Sloops and the Galleons (though exaggerated) look like historical galleons. Awhile back I tried to find a picture of ship that looks like the game's frigate and couldn't find anything close. It doesn't look like a frigate but it doesn't look like a galleon either. So it is probably a made up ship, but it looks really cool.
One thing to consider is that playability is going to trump accuracy every time in this game. I think that the developers wanted the ships to be easy to tell apart at a distance even in low graphics mode. This may explain why the frigate looks as it does. A realistic frigate may have looked too much like a sloop.
I also believe that the designers never intended these ships to be replicas of the ships they are named for but rather to be broad representatives of different ship types. For example sloops are representative of all types of ships that are built to be fast and maneuverable above all else.
By the same token all Ship-of-the-line means in this game is a really big, bad, warship. As long as these ships add new challenges to the game I don't care what they call them. They can call them minnow class ships if they want.
Remember this game is basically an interactive cartoon based on a fantasy adventure movie which was based on an amusement park ride, so put your suspension of disbelief in high gear.
EDIt: Eliza posted as I was writing this. Let me just say then, "What Eliza said!"