As above ... 0.030"
If you find the manual you will find it says 0.040" but all the plug gaps were revised sometime during the late 1980s'
You will not notice any difference but at 0.030" the plugs might last longer.
The correct plugs are Champion QL77JC4 ( or QL86C )
These are inductively suppressed plugs. Do not use resistor suppressed plugs such as NGK BR7HS-10 which are often listed as the equivalent.
it's a 1981 7.5 evinrude. What is the correct spark plug gap?