There's a dive shop in Le Marin. I'm not sure the name but I guess they do rentals, or the
charter company can put you onto a rental co. North or south, both directions are great for a few weeks. I'd lean toward north though.
Dominica is great for
diving and hiking, as is pigeon island off
Guadeloupe. Then up to
Antigua and Barbuda. The saints along the way and you could drop by Montserrat if time permits.