Did the vans have actual "TAXI" license plates on them? That assures a licensed taxi. If they were working inside the secure area of the pier, chances are they were legit. If they caught the van outside the secure area then all bets are off.

Taxi rates are pretty much set and if in doubt there should have been a dispatcher at the pier who would have provided the rate. An island tour can be more costly than just a trip to the beach and any destination beyond the initially agreed upon destination would obviously result in more cost.

It is always best to negotiate and agree to any fare that is not set in advance. Most of the drivers should know what the fares are and when asked should provide them to the customer.