I'm looking at my Mariner/Mercury Service Manual 90-878079 Jan 2000 for Models 135 Carb, XR6/MAGIII, 200 Carb,150 XRI/175, & 200 XRI, serial numbers OG960500 and up. Page 1A-3 in Specifications and under Ignition System states the following:
Type - Capacitor Discharge
Spark Plug Type - NGK BPZ8HS-10
Spark Plug Gap - 0.040 in. (1.0 mm)
Firing Order 1-2-3-4-5-6.
However, my son has an older Mariner that calls out the BU8H.
If in doubt, check with your Mecury parts dealer, and based on your serial number, he should be able to tell you exactly what plug is recommended.
Russ