boat cranks but wont startr
If the pump doesn't prime when you turn the key from off to on (not all the way over to hit the starter) then your problem is in the wiring. It should prime for a couple seconds then stop until you start it, then it stays on. This would explain why it shuts off when start is released, 12v to the fuel pump is being cut too. Somewhere down the line power to the fuel pump and likely the ignition are wired incorrectly.
boat cranks but wont startr
Should be somewhat straightforward to trace down with a multimeter and wiring diagram.