96 75hp 2 stroke runs rough at idle

My merc runs great when Give it throttle (smooth as glass) but at an idle she is as rough as can be, no problems starting fires up first time every time, also and I dont think its related but while I was flushing out the motor yesterday after running in saltwater for about five hours the water coming from the impeller was cool to the touch I thought it should have been warm bordering on very warm. Do all 3 cyl mercs run rough at an idle?
Thanks for any input

No, they don't run rough.

It could be caused by numerous things -

Old gas (more than 2 weeks old)
Old sparkplugs (more than 2 years old)
Air leak in the fuel line somewhere
Clogged idle jet(s) in one or more of the carbs
Bad carb gasket(s)
Bad powerhead base gasket
Bad intake or exhaust cover gaskets
Chipped/bent/broken reeds
and the list goes on

Start with the basics - give her a tune up unless it was done this year.

New gas, new plugs, check/clean the fuel filter.
Install a water separating filter inline if not equipped.
Run some Seafoam (or similar) in your next tank of gas.

If that doesn't clear up your issue, pull the carbs and clean them out and then take it from there.

(should have added)

Your "cool" running could be a toasted thermostat which can also cause a crappy idle - test or better yet replace the thermostat (under 25 bucks) and if it's original, it's due anyhow...

thanks for the input and advice, I just rebuilt the carbs new plugs, changed the fuel filter rebiult the fuel pump, replaced the water seporator and draind the tank and put in fresh fuel with additive I,ll try the thermostat next thanks again. From reading your replys to online postings I was under the impression you didnt think very highly of seafoam.