You could go to shore in soufriere and get a taxi. Or you could get a rental car. The buses don't go to the base of the pitons, they stop a few miles short.
Or you can go to shore at the beach at the base of gros piton and get a guide from there and walk up.
Regardless, if you arent already aware, be aware this is a high crime area to leave your boat
for the day.
Lock both your boat
very well. Do not hire one of the guys coming to your boats by dinghy
to take you to shore or to be a guide for the pitons, some of them do that just so they know when you are away and can break into your boat. Even if you're careful, the chances of an unattended boat being broken into are still significant.
Do not pay the boat guys for a mooring
ball, they belong to the park and you'll just end up paying twice.
There was a fish
guy and a fruit guy when I was there a month ago though who I appreciated that they came to the boat.
If I were you, I would instead anchor
in rodney bay and find another group interested in going and split the cost of a rental car.