Originally posted by Draconifors:

Using only wood and core wood (stone was not yet unlocked), my friends and I built a bridge tall enough for at least the karve to pass under, across a river. It was a huge challenge to get the top roof pieces to hold, and took more than a little tearing down and rebuilding to accomplish. But, it was super fun, and now the bridge is there to always remind us of the project.

I know the feeling. Had a hard time building a covered dock for my boat as well because of the height/structural limitations of only wood.

My advice to the OP is to try to make it in relative shallow water. If the water level is lower than half of the pole, it wont rot. The hoe and the farming tool (cant remember the name) will be your friends.