boats?

i have a mercury 25 outboard engine,it has 2 spark plugs.2 boatmen have looked at it,and are not certain?if it needs 2stroke oil.there is no model number anywhere to be seen,the engine is a pull start,and a ignition key start,one of the boatmen reckons i should put 2 stroke in the petrol to be on the safe side?i have done this,but i have noticed,it gives off a lot of blue smoke,even after its warmed up,have i done the right thing,do the plugs need cleaning?? help would be appreciated. regards