John,
Unless things have changed in the last few months, your statement about the $5 is incorrect. We heard from many of our guests that taxi's quoted them $40/BZ from town to Caribe Island. That is less than 3 miles.
So yes, they do run all the time, but make sure you ask them how much, and make sure they mean for the entire group. (I had one very upset taxi driver tell me that it was $10US per person -- with 5 other people-- to Caribe Island.)
PS: I will certainly put in my kudos to ElDivino at Banana Beach-- breakfast there is WONDERFUL!!!!!!!!!!!!!!!