i've had a few boats with some big problems lately-the one that almost sank due to clogged cockpit scuppers and this one here...
problem here came from a missing "o" ring on the fuel fill cap,this allowed water to enter the tank and wreak havoc with the engine...i pumped over 35g of this "crap" outta the tank...now,i know some of you guys are gonna go by the "ethanol thing","oh that fuel is a big problem"..."blah blah blah"...truth is,water in the tank with any fuel is gonna create problems in any fuel-common sense huh
this is a yamaha vst,what you're seeing is rust,rust on the fuel pump-this is an "ox66" engine




this is the "bowl",note the rust...







note what looks like a float for a carb,in the first pic...this allows fuel to enter the vst,the assembly has a needle,that needle is made of aluminum,a common problem with this is,the water in the fuel causes a galvanic reaction between the aluminum needle and the bronze seat,the end result is the needle sticks and wont allow fuel to enter the vst,end result,the engine will not run...

moral of the story here is,check the little things,the simplest things can and will cause the biggest problems...