Not that simple. Dams and canals aren't just subsides, they're system efficiency multipliers, much like the Interstate Highway system. Sure, we all pay for such things, but the benefits to transport or farming efficiency reduce end consumer costs significantly.
But, as we say in the water industry, Whisky's for Drinkin', Water's for fightin' over!
You have to ask yourself why we subsidize certain kinds of infrastructure, and whether that rationale applies equally to transportation as to dams. The answer is it doesn't.
There are two basic concepts in environmental economics: positive and negative externalities. A positive externality is when someone benefits from something they don't pay for. Markets under-produce things that create positive externalities, because the price does not reflect that externalized benefit. You correct for positive externalities with subsidies.
A negative externality is when someone is harmed by something they do not get compensation for. Markets over-produce things that create negative externalities, because the price does not reflect that externalized cost. You correct for negative externalities with taxes.
Transportation infrastructure creates positive externalities. People benefit from a train line besides those who pay to ride it. Dams create negative externalities. The environment and people downstream are harmed by diversion of water, but they don't get compensated for that harm. Consumers buying produce is not a positive externality, because they pay for that produce. Water is an input to production, and the benefits accrue pretty much only to water users, or those who purchase a product or service from water users. If we stopped subsidizing water, the prices of California avocados would go up, but those prices would be efficient prices that correctly reflected the true cost of growing produce in a place with scarce water.
Eh, people also benefit from water management. The alternative is pouring all that fresh water into the ocean.
You'd be paying for these guys to do it less efficiently, or paying more to the other guy who farms it in the Midwest or S. America because he doesn't have the competition.
But, as we say in the water industry, Whisky's for Drinkin', Water's for fightin' over!