There is no exact definition of "marine gas", it's a general description and could possibly include, no road tax, no ethanol, premium fuel (high octane), or just what they sell in the pump at the marina.
Modern small 4 strokes have problems with any fuel due to having very small jets and being tuned to run as lean as possible, plus not being legally adjustable by the servicing dealer or you.
Any debris in the fuel, water, dirt, gunk, bits of dissolved fuel line (which the gray Merc fuel line is known to do) can plug or limit the flow through these small jets and/or the no longer adjustable low speed needle. There is a plug over the needle that can be removed and a screwdriver can be used to adjust the screw for easier starting and possible running issues.
This needle is illegal for the dealer and you to adjust (EPA), this law was put in place 1997 or 8, and then over time the emission regulations were tightened and the motors were required to run leaner, which may result in hard starting and a poor idle. If the carb you have happens to run a little too lean for your motor the only option is for the dealer to put on a new carb and hope it helps, they can't adjust the one you had.
The solution is to drill out the plug and turn the screw.
Now if this motor did run well at one time it's just something partially plugging the low speed circuit in the carb.
Also, these Mercs tend to be hard to troll with on small boats because of the lean jetting, it can cause the motor shake a little more, plus it can be hard to control the RPM's at very low speeds. Slightly richening the idle mixture screw can help with these problems.