My Mercruiser I/O sputters, sometimes to a stall. It revs fine, but terrible idle. Any solutions out there?

It's fuel........and yes water......and maybe old fuel (from last year?) You didn't tell us which Mercruiser you have, but the "winterization procedures" , the rule for 30 years have changed. E-10 fuel absorbs water times the rate of fuel three years ago. Then it will under go phase seperation.....and it goes "stale" much faster. Bottom line.....replace all filters, add stabilizers when you purchase your fuel. Run the current fuel out, don't add fresh fuel to bad.......rebuild the carburetor. If you don't have electronic ignition, do a tune-up. Alcohol based fuel "dry" such as Prestone Water Remover makes the problem worse. It's the Alcohol that is causing a lot of the problems. So, to sum up, don't let the boat sit with fuel in the tank or carburetor . Buy and use what you need. Change filters more often, and add the new types of fuel stabilizers when you fill up.

You're right about the fuel filter, but I'd change it anyway if you think water even MIGHT be an issue.

It sounds more like your carb might have dirt in it or just need to be adjusted. There should be a high-speed and a low-speed sdjustment screw, right next to each other, marked "H" and "L"; the one you want to mess with is the "L". Make note of how many turns you move it...Try backing it out first, say 1 turn, and see if that helps. If not, try 2 turns...if it gets worse, turn it back IN the same amount to get it back to where it was, then try bringing it IN some.