Fuel shutoff valve?
 

After having to disassemble, clean, and rebuild my Mercruiser two-barrel carburetor, I wondered about the possibility of installing a shutoff valve in the fuel line, close to where it enters the carburetor. There is one on the line from my snowblower fuel tank, and when I've finished using that each time I close the valve, leaving the engine running at a fast idle until it runs the fuel out of the carburetor. Would that approach work at the end of boating season to help with the problem of the (considerably larger) boat carb fouling as the gas left in the bowl slowly evaporates or fouls due to the ethanol in it, treated or not?

While your plan may be helpful, good results can also be had using StaBil in the tank.

Why
 

I don't know the age of your snowblower but I am wondering why you want to empty the carburetor after each use. The gas keeps rusting away and also keeps gaskets wet. My thought would be to leave the gas in there, and as was said, put Stabil in at the end of the season, still leaving it wet.

I do understand the safety of a shutoff valve but I would use it only to limit the amount of fuel that could spill to just the fuel that was in the carburetor.

Regular use of marine Sta-Bil (blue formula) and you're good to go. I also like to throw in a dose of Sea Foam now and then in my recreational vehicles.

On a lot of boats the shut-off valve is located on top of the fuel tank. Commercial boats are required to have a shut-off valve external to the engine compartment in case of a fire. Boats with fuel injection do not need a shut-off valve.
To keep the carburetor clean use marine grade Valtek, Startron, or Stabel every time you fill up and especially when you store the boat.

If you're comfortable doing low voltage electrical work, how about putting in a switch that kills power to your fuel pump? My Mercruiser engine has an external fuel pump, so the installation was really simple.

The fuel pump on a carbureted engine is typically mechanical, not electric.

Kill two birds...
 

You could install an outboard type of quick-connect link. This will allow you the option to use a portable tank for a variety of reasons. You can also drain the line to the carb with less spillage. You may want to use a rag or a bowl to catch the drips for safety reasons.

Good luck!