Xiamen Transportation

Go to the main Ferry terminal which is just across the Zhong Shan Road.
Note that there are two separate 'fare' systems.
First option:
You just walk through the gate without paying, and take the 'normal' service ferry which will get you to GulangYu in about 5 minutes (ok, maybe 6 minutes), and no payment is required. You only pay upon returning back to mainland (RMB8 price as at Dec 08). So it's a one way fare.

Second option:
This is some company running the service which will "circle" around Gulangyu for about 40 minutes. This option is good if you want to see some 'sights'. On board crew will introduce various 'sights' along the way. The fare for this option is RMB 15 (price as at Dec08), and it's free on your way back to the mainland. Remember to keep your ticket.
"Value added services"
1. When I took this option, the moment you climb on board, the operator 'offered' binocular rental service which you can opt for about RMB15. It's up to you if you want to take it. I don't think you can see much with the binoculars unless you're planning to zoom into someone's bathroom window.
2. They also offered to sell you tickets for RMB 80 (discounted price, normal price is RMB 135 for about 5 fixed attractions - i hope i'm correct) for some of the attractions in Gulangyu, claiming that you can't get that price in the island. You can actually get that in the island itself, near the ferry terminal. This option is good if you want to go to ALL the places listed on the ticket.