DCockey is correct: if you build a boat for your own use with no intent to sell it then you can do what you want, but you may have to register it with your state (depends on the type of boat) so keep all your receipts for materials so you can prove what it cost to build, because most states tax you on how much it cost. In this case the cost of materials. The state will give you a Hull ID number for the boat as well as a registration number. (PS: in Pennsylvania they will even give you a capacity label for it.)
However if you decide to build and sell, you become a manufacturer, and you need to register with the USCG to get a Manufacturers Identification Code, a one page form and no fee, so you can assign Hull Identification Numbers to the boats.
If the boat is an outboard, inboard or manually propelled boat (but not a canoe, kayak, or sailboat with no motor) then there are safety standards it has to meet. You have to certify it meets those standards. This is much simpler than it sounds. You do this by putting an HIN on the boat, and a label that says you certify it, and a capacity label if the boat needs one. See
http://newboatbuilders.com for all the rules and regs and who t contact. Also useful is the USCG Boat Builders Handbook at
http://www.uscgboating.org/regulations/boat_builders_handbook_and_regulations.aspx
The USCG will even test one of your boats (assumming you built more than one or two) for free to ensure it is in compliance, and if it isn't will help you correct it.
As for liability, you are responsible for the boat being in compliance with Federal Regulations for 10 years. But at the end of ten years you are not completely relieved of responsibility. You are also responsible if the boat "contains a defect that causes a substantial risk of injury or death to the public", and of course some hotshot lawyer can always find something to sue you for.
If the boat is an inboard or outboard there are also EPA regulations for the fuel systems but these are easy to comply with and anything you need can be purchased from suppliers. You don't have to design it, just install it.
All that sounds pretty scary, but in fact compared to the number of boat builders in the USA (about 4000) there are damned few law suits and they are usually aimed at big time manufacturers like Brunswick (US Marine, Sea Ray, et al,) Bombardier Recreation Group (they own 6 or seven companies) Larson, Chris Craft, Wellcraft etc, that produce 80% of the boats in this country and have the deep pockets. Small time mom and pop manufacturers are rarely sued and if they are they usually just file for bankruptcy and the suit dies.