Inconvenient Wife by Natasha Boyd

All Beau Montgomery, one of Charleston's most eligible bachelors, wants to do is build boats. He has no interest in marriage whatsoever...until it means he might not get his inheritance and he can kiss his boat-building dreams good-bye. But the hungry hordes of Charleston debutantes looking to score a Montgomery scare him shitless.

Gwen has loved Beau Montgomery since she used to run barefoot around the marina as a scrappy preteen tomboy. Knowing she'll never fit into his high-society family, she decided long ago she'd become his indispensable best friend rather than not be in his life at all. When he tells her he needs to marry in order to fulfill his dreams, she panics that he might actually meet someone, and she'll lose him forever.