you need a Bill of Sale notarized in the country where you bring the boat in from. based on that price shown there they do a valuation of the boat to decide if the shown price is acceptable or not. they will never decide on any Lower price after valuation, but they often tell you that you made "a great deal" because by their understanding your boat is worth much more than you paid for it, lol.
20% and thats all? the ITBIS due alone is 18% and the ITBIS is the lowest of the 3 applying Taxes for a import. who told you that did forget to tell about the other involved taxes, as it is 3 taxes/fees you pay for importaing a Boat. they total under the line on well over 50% of the Boat's evaluated worth. for example a boat evaluated to be at the moment of arrival in the country worth $100KUS$ will cost you with all fees together at the end 50-60Grand extra for the import. if you are a US Citizen and bring a boat from the USA, do Not Import the boat, just bring it in for recreational purposes and keep renewing it's US Matricula and US insurance every year in the USA, so for your private use of the vessel you are only required to pay the yearly(can also be done every 6months) Fees for the "Permiso de Navegabilidad Recreacional".
just you would not be allowed to use such boat commercially/for tours/with tourists/rent it out/sell fishing tours etc etc, it would be for the private pleasure only, recreational use.
Mike