Have you checked the fuel lines going into the auto gas/oil mixing system(VRO) for debris, and the plastic fuel distributor that takes to one output fuel line from the VRO and splits it into three lines, one for each carb and one for the electric primer.
Inside the water pump housing there is a small groove that connects the impeller area with the outlet hole. If that get clogged with sealant while assembling the steel parts inside the housing, the pump will not prime while in a non-pressurized water environment.
Gentlemen, I just came across this listing for a 1978 Islander 22' on the denver craigslist http://denver.craigslist.org/boa/1326455185.html
Don't know anything about it, just giving any of you Islander fans a heads up.
Thank you all for the terrific info in your posts. Thanks to North Beach...