Vapor lock is invariably a temperature issue. Basically, I'd look for a point on which the gas line gets too near a heat source. Beyond that, I'd agree w/heather: the carb's secondaries may not be working/aren't kicking in totally. However, this should still allow her to idle...
First of all...your motor is the same as that in a mid-70's car. However.....there are some very important differences.
I am assuming you want to approach this "on the cheap"...but you gotta know what ya don't know.
First, I would use the Internet to learn about marine engine cooling systems...