When cold, turn the key on dont crank it wait 3 seconds, turn key off, turn key on again wait three seconds crank engine.
(this cycles the fuel pump a few times)
if it starts right up and stay running the fuel rail pressure is leaking down

If still stalls, warm it up, then remove the flame arrestor, check for crud build up around the inside of the throttle body
Start with carburetor cleaner, clean butterfly, throat and the IAC (Idle Air Control) ports.

let it cool and check again

 

Thank you for the quick reply! I will give it a try when we get the boat in the water in 2 weeks.