Just look at the stats of the ships from the ship vendor in the highest level area you can go into. Add the components that fit that ship from the parts vendor standing in the immediate vicinity. Without spending crowns, that is the best ship for your level. The crown ship will of course be better. Just compare the base stats to the ship you currently have to decide.

Dont let ship size and style be your determining factor... its all about the combat stats.

With all that being said, none of that may actually be your problem. Its likely you havent spent much time on the ocean killing ships for nautical or doing the side quests that award nautical. If your level is high but your nautical level is low, you will be constantly facing enemies for your level but trying to fight them with your nautical level. That almost always results in you getting boarded since you dont have the ship to deal with them.

As your nautical level increases, you will be able to get better ships and parts.

If that is the case, I recommend using whatever your best ship is and sailing back to areas youve already completed. Start sinking ships (not boarding) that are grey to your current level. The level of the ship needs to be close to your nautical level. This will give you nautical experience. Bring the two types closer together and you will soon be able to sink ships your own level.

Bison Frigate?

The Bison Frigate is the best ship you can get before you enter Port Regal. Once you gain access to Port Regal, you can buy the Royal Navy skiff, which will be more powerful than the frigate you have now.

If you are getting boarded way too easily, I suggest you try to get your nautical level up. It seems to me that higher nautical levels make broadside combat easier.

Also, while having the Bison Frigate makes you very strong, you can still suffer damage quickly from the ships in Cool Ranch. So, consider having some high accuracy/low damage weapons instead of low accuracy/high damage weapons. I had the Bison Frigate for a very long time, but got slaughtered if I used Horn of the Gorgon and Fire drake spells and constantly missed.

If you have a buddy with access to Tumbleweed, you can save some gold and teleport to your buddy. Over there, you will be able to buy the Bison Galleon, which will be more powerful than the Royal Navy skiff.