The required spark plug for a 50 hp outboard engine would depend on the make, and year model of the engine.
The spark plug gap for a 2003 Mercury 125hp outboard motor is .040"
Any motor should get spark with or without gas in it. The ignition system works off electronics and does not require fuel for it to work. Yes you should get spark.
.030
spark plug gap for a 1996 mercury 30 hp outboard motor?
What is the spark plug gap for a 9.9hp mercury outboard motor
The spark plugs on a 1988 model, 70hp Johnson outboard, should be gapped to .030in.
.030
.040
.030
Ngk bpz8h-n-10
NGK bpr7hs-10
.035