I found this today while browsing Royal Caribbean's website...
http://www.royalcaribbean.com/findacrui ... rtCode=TPE
I called and apparently it is possible to jump on the cruise midway and get off back at your original port. I've seen people in the Caribbean do it all the time so I wanted to make sure. They said the availability of rooms is very limited for people who get on midway and there is a surcharge, but it seems like a decent option for going back and forth to check out some places in Japan leaving from Keelung (Jilong) considering how expensive it always is to fly to Japan from here. They also informed me that there's a new agreement that was reached between Mainland and Taiwan recently and they will begin cruises going back and forth soon across the strait (THANKS KMT)! Hell, their Asian cruises are cheaper than a cross-strait plane ticket, not to mention my super VIP status at Royal Caribbean where I get showered in free shit. I'm 2 cruises away from FREE DRINKS. Anyone checked out the Singapore cruises? US$400 for a 5 day cruise around Malaysia! It's an older ship (Legend class) but still that's not bad price-wise. I usually just eat the whole time, get my money's worth in prime rib.
I would choose a Royal Caribbean cruise ship over to Xiamen or, even better, Shanghai over that ferry which I can only assume is marginally better than the Green Island one any day.
I'm SO EXCITED.