I'm wondering if condensation or wet conditions could be a factor.

Do outboard motors need special 'marine' spark plugs or will regular spark plugs work?

I'm not aware of any specialized spark plugs for outboards. Though I haven't actually looked for them either. So there may be such a thing. I purchase my plugs from an auto parts store.

I'm not aware of any specialized spark plugs for outboards. Though I haven't actually looked for them either. So there may be such a thing. I purchase my plugs from an auto parts store.

I have a 60's era Evinrude that takes a marine plug. Not really sure what constitutes marine. I had a 88 Johnson that I believe I put standard plugs in. And both my mercs have been professionally maintained and use whatever the official Merc plug is

A lot of marine engines use a "surface gap" plug for clearance reasons. Although they are not "marine" specific, they are the correct plug for the engine. I run NGK in my Merc. NGK or Nippendenso are good brands but the most important thing is to get the correct plug for the engine. You have to have the correct length, thread, gap etc...

Normally, what makes a marine plug, a marine plug is it has a stainless steel base.  What you want to put in your outboard is what the manufacture recommends.  I don't even like to vary brands names with them, if specify a Champion plug, I use a champion.  Sometimes, depending on the particular engine, I will go down on heat range if that motor is going to be run a lot a WOT.  Other than that, I stick with what the book calls for. 

 

If for some reason I have to substitute a brand, I will use NGK's.  I have had better luck with those matching the brand it calls for.  The brand does make a difference, I've put what other brands claim are the right plugs and it would barely run, so from years of lesson's learned. I stick with the manufactures recommend brand name.