The number of days you are granted upon entry can depend upon your immigration official, I had a junior one tell me he was only allowed to do 20 days and I'd have to get the senior one to give me 30. I've also extended beyond the 30 days twice so far, once in Road Town and once on Virgin Gorda. It was not easy to do in either case (the first time my rudder
had broken and I was awaiting parts
and had to produce documentation
of all type, the second time was frustrating as after waiting over 4 hours in line in Road Town the gent told me that I had to come back the next day (and wait another 4 hours) with an official printout of my airline tickets).
It is much easier to get the ferry
to the USVI and leave the BVI officially, then return on the same ferry
an hour later.
The temporary importation of the boat is normally required, but even there the officials may or may not check to see if you have paid for it. Considering the financial troubles in the BVI and the fact that they've raised the fees
for the immigration forms (I had to pay $1 for the form last week instead of the customary $.25) they will probably be enforcing the temporary importation fees
more closely this year.