And...I land on this one again. I can never remember which books might have boats in them. Although I just recalled that since I've reread Hogfather, my next Discworld reread is Jingo, where they travel by ship. Are we being technical here, or does it not actually have to be a boat? Otherwise I've come up Recovering Apollo 8, River of Stars, or Silver on the Road for "the letters in the title can be used to spell 'River'". If I'm allowed to reuse R's I could consider Sundiver as well.
Anyone care to weight in here?