Re: mercruiser 3.0 rough idle
It's great that you are using 93 octane, dont think its necessary in that engine, but it wont hurt it in the least. Sounds to me that you need your idle mixture adjusted and possibly your choke if you have one ( TKS Carbs ) dont have chokes.
Before Starting the first time for the day, do you pump the throttle ? how many times ? I would say that if the carb is properly set (choke or not) 1 time is enough to start the engine, it should then idle without issue. This sets the choke if equipped and puts a bit of fuel into the intake manifold so the engine can fire.
If the idle mixture setting isn't right, you will have 2 basic things that will happen, it wont be "easy" to start and it wont idle properly, could be either in gear or in neutral depends on how off the adjustment is. The best way to set the idle mixture is in the water with the engine warmed up and in gear. with the boat tied to a dock or trailer backed into the water is best. adjusting it with the boat on land an runnign form a garden hose is a little more difficult though it can be done, but puttng in gear isn't helpful. You do want to have your drive lowered as much as possible.
It could be time for a carb cleaning & rebuilding.