You could make the same argument for the national interstate highway system. Would you like user fees there? Why should those who dont drive on them have to pay for it? Because a user fee would cripple the entire system and effect the economy negatively.Right now this is being paid for primarily by a large number of people who do not document their boats (the general tax payers). I can't see why you would expect them to pay for it.
How about airport user fees? That has been discussed in the aviation community for decades as liberals look for ways to fund all their increased spending. But it has been shot down time and again when it is shown the actual benefits to the economy at large by keeping it federally funded and not having user fees.
There are many other examples, but I think you get the idea.