Firstly, welcome to ribnet!
There are plenty of online cross - make equivalent charts. Thatr wil lat least tell you if they are similar.
Spark plugs are rated on a few things, including their ability to conduct heat away. If the two types are similar physically, then in theory the "wrong" one would let your engine run, but may alter the performance once it warms up. (Heat dissapation etc) Some also carry internal suppression, so you also may find your radio starts buzzing at you.
If they don't turn out to be identical, but they are physically similar, then there are two ways of looking at it:
1) Carry 2 sets of spares so the correct plug goes in.
2) live with possibly reduced performance on one of your engines until you can get back ashore for a new one.