Originally posted by redgreen999:

There is a knack to boat placement.

Exactly this. And leaving it in shallow water alone can be reason for it slowly going down.
A mere boar can and will pick a fight with your vessel if it sees an opportunity.
Proper vessel placement is a learned thing sometimes it takes some trial and error to figure out what it takes.
What biome have you been leaving your boat by? Cause thats important.