Yeah Adventure Antigua is a great round the island trip. They also pull into Stingray City where you can swim and feed the stingrays. It's a great way of seeing the island and the wonderful beaches, it's well worth the money, I have been on it many times.
As for Darkwood there are plenty of Taxis available at the port, just let the driver know you want picking up at the time you desire and they will return. It takes about 20 minutes to get there from the port.