As far as I know displacement = weight. Certainly the weight of the boat
will
displace the amount of water with that same weight (hence the term
displacement). That said, for documentation purposes the Coast Guard refers
to
a tonnage (don't recall if they call it displacement) that relates only to
boat
volume, not the weight.

Although not as common as a reference to cubic volume, calculating the weight
of a vessel is an uncommon but still correct use for the term tonnage.

When speaking of volume, gross tonnage is the total amount of space contained
within the hull, divided into 100 cubic foot portions. Net tonnage is the total
volume, less the space taken up by engines, generators, fuel tanks, etc.

A boat with 1000 cubic feet of open area where cargo could, theoretically, be
carried
would be rated at 10 net tons. Would have nothing to do with that vessel
weighing 20,000 pounds.......which unless by sheer
happenstance it probably would not.

Then it gets a bit trickier yet. In many cases a "ton" refers to a "long ton"
of 2240 pounds.

Undoubtedly some young woman once who got pretty drunk, was slurring her
speech, and started fantasizing about looking for man who was a good kisser
with a "long ton"......
have to wonder if she would have thought a long ton was so desirable if it came
attached to a guy who weighed 2240 pounds. :-)