I would definately recommend a LAND taxi over the water taxi. We did that once and it was enough. Far to many people crammed into a space that had it turned over, we would have had no way to escape.
I would be surprised if the taxi ride for 4 people was more than $10. When we did The Dig (the aquarium) it was $25 a person. Sounds like they have gone up. However, if you are on a cruise that arrives early and you get to The Dig before 8AM (that is when the kiosk opens) ...just start walking through it like you own the place.
We paid the full price the first time....however, we got there at 7:30am the next time and the kiosk was not open...so we just walked through on our own. If anyone asks why you don't have a wristband ...just tell them the truth..they were not open!! No one ever asked us.
As interesting as it was, I thought $25 was ridiculous. I can't see paying $30. IMHO You can see a lot of the aquarium from the downstairs restaurant.
As for shopping, there is plenty around the ship area. The Straw Market for folks that are into that....and several streets away from the dock have shops. John Bull has a great selection of Rolex watches. Their prices are not as good as Kirk's in Grand Cayman...but they have the best selection.