you can take the public bus for 2.50 USD from the bus depot which is a 5min walk from the cruise port. Bus 1WB will take you there. it is like a 30-40 min ride and it stops right in front of the center. just tell the driver where you are going.
Now coming back, it seemed like we waited for 20-30 mins for the bus to come by. by the time we got on, there was us (4 ppl), another group of 5, and 2 singles all boarding at the same time. if the bus is full, you may have to wait for the next bus.