Have only used them on St Maarten and they were extremely close to where we docked. There were plenty of them 9and as they took quite a few passengers at once didn't have to wait long) As far as cost can't really remember other than it wasn't expensive. It was in the region of $5-10 return. On some islands you will be able to walk to the beach and in others a conventional taxi or shuttle bus will be quicke.
There is a regular shuttle service provided by locals in Grenada from just outside the terminal gates around to Grand Anse beach. You can see it from the ship. At the beach end they just run up onto the sand so you will get your feet wet getting off. It's a nice sandy beach with plenty of bars and restaurants. Can't remember the cost but they are eager to get your business.
Re Granada, it was $3 each way, we always head to the right along the beach to Lasy Days. They should have now completed the new landing stage at the beach .