You might take a look at
This one from the Camper Clinic catalog (sorry, they won't let me copy the pic.).
A lot cheaper than marine units, but probably a lot
cheaper, too... Camper Clinic has a couple different models to look at, if you are so inclined.
If you are looking for a portable solution, I saw a DIY field sink that I thought was pretty cool. The pump idea they used could certainly be adapted to a water can.
The foot pump is simply a marine bulb fuel pump (like you use to prime an outboard) sandwiched between a couple hinged boards. The bulbs are pretty cheap at Walmart...
The rest of the faucet could be a simple bent piece of tubing that somehow clips to the water can.