At the advice of my charter agent, we went with BVIsupco at http://www.bvisupco.com/.
There umbrellas are only $15/week for rental!! They have plenty of umbrellas and never run out, although they are not listed on their website. Additionally, they rent beach umbrellas as opposed to the canopy umbrellas which are preferred as you don't have to constantly move around as the shade shifts with sun movement. They also offer chairs, surf boards, dinghy ladder and all other type of equipment to suit your sailing needs. hey will deliver to your boat and secure all equipment prior to your check in. Fiona BVIsupco is a pleasure to work with and promptly responds to your emails or calls firstname.lastname@example.org or 284.346.1981. IMHO.