Re: "I'm told that I sing certain sorts of songs (ie unaccompanied sea shanty) better than guitar backed singer/songwriter sort of stuff."

Don't worry about it. That was a silly thing to say! Find some songs that feature whatever it is about shanties that brings out the best in you, and sing them.

With the millions of possible songs out there, you can find songs that are good for you, whether accompanied or not.

Possibly your critic has heard only the "singer/songwriter stuff" produced by the pop music world in the last 20 years. Most of it would make anybody sound bad.