Have you kept up with general maintenance such as fuel filter, tune-up, (Spark plugs, cap, rotor, etc).?
Boat new to you or did this start happening recently?
Boat run normal once started such as top RPM reached?
Does it do this at every start-up hot or cold while? (On the water every time or just first trip after trailering the boat to launch ramp, etc.)
Could have a leaking fuel pressure regulator, leaking fuel injectors, aging fuel line between tank and fuel filter, fuel filter not remaining full of fuel, weak fuel pressure from pump, sticky fuel tank anti-siphon valve to just to name a few.
Testing both the boats and engines fuel system may be needed to try and see where the problem seems lie.
Next time before cranking the engine to start, try turning the key on for about 5 seconds, turn key back off for 2 seconds, turn key back on for 5 seconds then crank engine to see if it starts right up for you.