I can give you a few suggestions:

1. Iboats.com have a great forum and a great group of posters:

http://www.iboats.com/cgi-bin/ubb/ultimatebb.cgi

These are also active:
http://www.goboatingamerica.com/cgi-bin/ultimatebb.cgi

http://cruisersforum.com/


http://www.boatohio.com/cgi-bin/forums/dcboard.cgi

2. We have been growing a regional boating site here
http://www.ohioboating.net/b2/index.php/a

with a forum that we have been putting topics in but haven't really
attempted to push it, so it basically a stage waiting for the first
act.
http://www.ohioboating.net/forum/

If you would like to help us grow a community, you are more than
welcome. Trust me, all topics will be monitored. There will be a forum
that can accommodate moderated off-topic issues but it will never be
allowed to deteriorate to the idiocy some of these sites allow. Point
is, you shouldn't have to wade through political bickering to have your
boating questions answered.

As I say, we are growing it - we have one final stage to complete
before we start pushing the entire site but the main objective is to
fill it with the voices of boaters in the region; we will offer hosting
for online journals or blogs,galleries, etc.
If you like to be able to shape the site and be part of growing the
community, you are welcome to join in.