Even if a bank/finance company doesn't require it, documentation provides a record of loans, title transfers, etc from the federal government. Financiers like that a lot.

As far as the display of state numbers on a federally documented boat is concerned, it is against federal law. The USCG can cite you for the violation. The state can require you to register the boat and pay the fees, but they cannot require you to display the numbers. They can require you to display a registration/tax sticker.