Put the summary of the bug you're having here:
I placed a boat in water 1 block deep sailed across the area and got out. I wanked away and came back about one minute later to find the boat about 15 blocks in the air. Does the same with fences next to water.

Steps to Reproduce:
1. place boat in a pool of 1 deep water
2. sail for about 30 seconds
3. get out, walk away
4. come back to boat
5. if boat isn't in air create a cobble stone wall next to water.
6. place wooden fence on cobble
7. place boat against cobble by sailing to it
8. wait