I live aboard a houseboat (dual stern paddlewheel) in a Marina and sell housboats on Lake Union, that being said I have obtained tons of info over the past few years helpful to potiential buyers of houseboats.
As John says, Houseboat are a challange to finance, however their are lenders, such as Frontier and will add another not mentioned Alaska Federal Credit Union where I was recently quoted 6.3% int. rate, 20% to 25% down at most, on 20 year mortgages.
This is for houseboat style liveaboards in either a month to month rental slip at a marina, or a condo style where you own the dock space. Depending on the length of your houseboat, dock rental and condo space fees can be similar.
Insurance can really vary from provider to provider, so you really need to research them. Insurance can be very expensive. It is usually offset by the fact you aren't paying property tax in rental spaces.
A really usefull site - http://www.seattle.gov/dpd
Look for CAM Client Assistant Memo 229. This will help you with Floating Home, House Barge or Vessel info. Good info!
An inspection for a houseboat is called a Survey which is the inspection and appraisal all in one. Very extensive and requires a haul out to drydock. This expense is to the buyer and sometimes can be negotiated. The haul out fee is by the foot $14.00 to $16.00, the survey can cost $450. to $700 depending on the size. Getting the home to the haul out site is an additional fee and can vary. Your lender and insurance will require a copy of this and can require all critical items be repaired to insure, or loan.
The biggy, that you will want to negotiate is the Sales Tax, paid by the buyer, 9% and is not rolled into the loan. When BECU was doing these loans a couple years ago they would roll it in, they no longer do them.
I am not trying to discourage you, it is a wonderful lifestyle. Just be prepared.
Floating homes are considered residential loans.