Even though Coastal Express shows Tranquility Bay as a stop, their last stop north is Blue Reef, about 5 miles south of TB. You could ask them what they would charge to go to TB.
When the TB boat runs, the fare between TB and SP is US$20 each way. The TB boat will go to/from Blue Reef for US$10.
If you charter the TB boat to/from SP, it's US$100 for a small boat and $200 for a bigger boat.http://www.coastalxpress.com/schedule/