It is very easy and cheap to travel from St. Vincent to Granada or vice versa by ferry via Bequia, Union Island and Carriacou. Other islands can be visited as well. No booking is necessary.This is a great way to explore the Granadines. In some places you can hire a taxi boat, that brings you to one of the uninhabited islands.
In Union Island Captain Yannis offers daily catamaran tours to the Tobago Cays.