One note... You will not be sailing down to the Spanish Virgin Islands
from the BVI... it is UP and to the West. Usually going under St. Thomas.
The "this August" is confusing... It is already October... did you mean November or December? If so the trip from BVI to SVI should be reasonable fast but the return could be a pounding into the Christmas
Winds and sometime crossing Swells from the North if they make an early appearance.
As for a very portable GPS the Garmin
486 works great in the BVI with the proper Carib Chip and can be moved around very well even to your car when in the land mode. I've use one for the past 4 years and it is dead on everyplace I've been. You still need to keep an eye out. Lots of unchartered stuff around.