You need to dig deeper. The Chinese did and still do talk about slow and fast boats between destinations. This got picked up in American listings for transits in newspapers and travel agencies. This was particularly relevant for freight and passenger travel where there were cost differentials. Witness the following listing in the current Yantai travel guide. “There are seven slow boat departures daily to Dalian at the ferry terminal. A third class bed costs ¥114-160 and the journey takes 8 hours. An alternative fast boat (four departures daily) which takes only 3.5 hours at ¥170.” Loesser simply picked it up and incorporated into his song. 
Since the boards do not have the technology to link a comment to an article, it’s considered common courtesy to link the article in question.
I’m going to assume this is it. Now you’ll know for next time!
How do you know that the Chinese had slow and fast boats between destinations as early as 1948?
Welcome to the Straight Dope Message Boards, Blackspur, glad to have you here. If you have a cite for the existence of slow/fast boats prior to 1947, we’d be glad to modify the staff report. samclem and I wrote that. I confess, we didn’t look at whether there were slow boats IN China, since the song was about slow boats TO China, and we had Loesser’s daughter’s poker story. It’s clear that the reason the phrase became generally popular was because of the song.
I doubt current use in China is relevent to Frank Loesser’s lyrics. The quote is from Wiki travel so the fast/slow useage is by whoever made the entry on the wiki page and refers to the split between conventional ferry boats and fast ferries - hydrofoils I think in this case. As they didn’t have hydrofoils travelling to China in 1948 I doubt this is what Loesser was thinking of!
Absolutely no cite but I’d always assumed a slow boat was a cargo vessel carrying passengers as opposed to a fast passenger liner.
I’m quite sure this has nothing to do with these ‘slow boats’ and ‘fast boats’. In that song spending time with this other person is nice. Going to China will make it last long ( 'cause in a boat she can’t stray away ), which is better, and if the boat ( whatever type it is ) goes slowly, it’s even better. Maybe for a few bucks captain would take a detour or maybe say ‘oops, We forgot something, gotta go back’… just anything to make the moment last, even if it means sabotaging the engines.