This is an early Thomas Perry book, published in 1987, wow the year I graduated high school. A couple that are con-people, steal a half million from a gangster and take off to Caribbean to hide out. He finds what he believes is an uninhabited island on some old maps. They come upon a couple other people with shading pasts to join in. The island gets covered up by the ocean/sea at high tide so is not really an island but the main guy Harry sees it as an opportunity. They find disguarded stuff--cars, dirt, concrete--really anything they can find to build this up into an island. They keep at it for years & becomes a resort with golf course, baseball field, housing, a sort of downtown. They sort of get in with the UN and become a kind of country. They have to fend off bad characters too. A sort of goofy but what if this could really happen type of story.
There were no copies on PBS so I posted it & it got requested right away. I should be mailing off in next day or two.