There are some hoops that need to be jumped through in order for a UK citizen to enter the USA, by pleasure vessel, using an ESTA. You MUST have first entered the USA by commercial carrier (plane or ship) within the last 90 days and your entire time in the USA must be no more than 90 days. For example, you CANNOT fly from the UK to, say, SXM, then fly on to the BVI and then take a pleasure boat to USVI on an ESTA. You CAN fly UK to STT or Puerto Rico, fly on to BVI, pickup a pleasure boat and then sail to USVI using an ESTA.

Most of this is explained in mcevog's linked info, above, but thought I'd put this clarification right here where it is more easily available. And of course you should always check the latest situation as things change sometimes without warning!