Generally, the beaches on the south coast of the island are sandy, many with shallow waters. From the centre to the north the beaches become pebbly with extremely clear waters,  I would suggest buying some sea shoes if you have sensitive feet as it is worth exploring them too.
I am not a sand lover so I prefer the beaches of the centre and north of the island but is you travel with kids or have mobility problems you will definately appreciate the sandy beaches more, some of my favourites include, Ai Helis, Xi beach and St. Thomas