You can under the same conditions you would for a house. IOW, you
cannot claim a loss for a home if it was solely used as a house. If
you used a portion of the boat as a business, either as an office, or
as a charter, etc, then the portion of the boat that was used as a home
office would be capable of being claimed as a loss. IE, $100,000
boat, with 20% being used as a home office sold for 50,000. The sale
is then considered to be two sales, a sale of a home office and a sale
of a regular house.
The home office depreciated, so it was worth 20,000 (20% of the
original 100k). It dropped in value by half, so you can report 10,000
as an ordinary loss to the IRS. The main reason for this is the IRS
is trying to get taxes from people who's home office appreciate, so the
rules were setup to seperate out a home office from a regular house and
then nail people for the appreciation in value of the portion of the
home that was their home office.

If you haven't been doing something like this though throughout time
you had your boat (ie claiming a portion of it as a business or home
office), then you also can't deduct.

Doug