There is a 6sigma tool called the 'house of quality' that could be used. I'm sure there are some for downloading from the net. Here is an example
. You'd put the qualities you want in a boat in the first column, rank how important those qualities are to you in the second column and then list the boats you are considering across the other columns, ranking how well they address your needs as High (9), Medium (3) or Low (1). Total the product of the ranking x importance for each boat along the bottom row.
I did this (not 6sigma) using a criteria spreadsheet, it was very useful.
I also used a sheet with all of the equipment I would like and line by line upgrade cost if the equipment that needed to be added or replaced for each potential boat, including transortation/taxes etc. Another sheet in the same workbook for boat shopping expenses, etc, etc. Like I said it was very useful, it helped us a great deal as some boats would have cost a small fortune to equip even though it didn't appear so at first glance.
Even if the upgrades cost more than you think, it's not the accuracy of the final amount that counts, it's the fact that you're comparing apples to apples for each potential boat.