1. You can probably do up to 20 year loans.
2. Yes, you can write off the interest payments as a second house.
3. I would go see if your local credit union has something for you. BECU here in the Seattle area has options. Here is a link to some of their info. http://www.becu.org/pdfsource/loanratesheet.pdf
As you can see for a boat loan over 100k they can do 20 years but would require 30% down you could get a rate as low as 6.74%.
I am not sure if there are other good yacht loan companies.