Haven't done it in awhile, but use to be that if you just missed the boat at the Boardwalk, you could still catch it by walking over to the Swan, since it would have to make a stop at Beach Club first.
I know...I thought that perhaps I had wandered off to the Theme Parks board for a minute there!!!!!
Anyway....the boat starts at Epcot, goes to BW, then over to YC/BC, then to S/D and finally to DHS. If you just miss the boat at BW, you can easily walk to the S/D and get it there. The ride, from BW, to DHS is about 20 mins.
You can walk to DHS in under 18 mins. I normally do it in 15.