Hopefully cut runner will chime in.
Ask and ye shall recieve
There's no direct injection on that motor, just carburetors ("controlled fuel leaks" I call em)
My first two things to look at would be the fuel pump and carbs.
On the fuel pump, there's a gasket/diaphram in the middle with a tab that sticks out on top. Is it white or black?
Also, run the boat again, and right as it sounds lime its about to stall, press the choke for a second or two and pay attention to see if it keeps the motor from stalling or if it still stalls like it has been.
Going from e10 gas to non ethanol usually causes a few problems because the non E fuel breaks all the little stuff loose in the fuel system that was put in there by the e10.
To me it looks as iff that stuff has broken loose and is sitting in the bowls of the carbs, the motor starts for a second then the idle jet starts pulling fuel up from the bowl like a straw and starts sucking up the debris and getting clogged, resulting in stalling.
Or the fuel pump diaphram is ruptured and puring fuel in the crankcase and stalling it.
Did you notice an abnormal extra amount of fuel sheen coming out the exhaust when you were running it on the hose?