If you have saved up cash for the purchase of a boat, the question of how much boat you can afford comes down to maintainance, what else you could use the money on, and what you feel is reasonable. This is much more subjective.
43% debt to income is the highest ratio that most lenders will lend up to for a mortgage. This is important because you do not want your boat loan to get in the way of refinancing, moving, or buying a new house.
Most boat loans require a down payment. For instance, SunTrust offers boat loans with 15% down payment. Some loans are with 10% down payment on boats 2001 and newer. Usually boats under a certain dollar amount will have higher down payment requirements.
On a new boat, or boat over $50,000 you can get a 10-15 year loan. In some cases, you can even get a loan that is 20 years.