My colleague says around 9pm but you have to be there at 8pm to check-in. Note that the ferry goes Keelung - Dongyin - Nangan - Keelung or Keelung - Nangan - Dongyin - Keelung on alternating days.
I did the opposite direction sometime last year, took around 2 hours from Nangan to Dongyin, short stop of around 30 minutes and another 6 hours or so to Keelung. My colleague and me rented a business class cabin (4 beds), first class is also available (2 beds) - or economy (dormitory style) if you are on a budget. I think the business class was only NTD400 more on top of the 800 for economy.
They also have a coffee shop that sells snacks and drinks (I recommend the cappuccino) and there is an area to sit / eat. Or go up on the roof and enjoy the view, if weather permits. During my trip we had heavy fog but the sea was calm at least (kind of spooky actually).
Here is the link to the company that runs the service, supposingly you can get prices there and also make a booking (can’t see it, it’s all garbled in my browser): shinhwa.com.tw/
They got ATMs and just recently 2 shiny new 7-11s on Nangan island. Otherwise there are some local stores spread around, though they don’t open 24 hours (AFAIK). Don’t expect the same on Xijue island though.
I know that thread is pretty old but so is uniair.com.tw! It says I can only browse the website with IE 5.5… it’s impossible to select anything as the scripts are not supported in any other browser. How can I book tickets on that website? I tried to book via expedia but of course all flights are always fully booked…any ideas?