If I bought property that bordered on water, but did not

[Jackie1957]

did not extend into the water. could I put a boat in the water next to my property?

[Alwin Alcott]

depends... if it's public ( or set to public):  yes

                  if it's private property: no

 

if it is set to public use, it also might have auto-return enabled, your boat will be returned to your inventory in a certain time.

[Lindal Kidd]

Basically, no.  Unless you own the land (including land that is under water) it's bad form to put objects there.  Even if auto-return is not set, the owner of the land your boat floats on can come by at any moment and return your boat to your Lost and Found.

The approved way is to terraform your own land parcel and sink some of it below the water's surface.  Then float your boat on your own land.