| | RE: 2003 Mercury 90 HP Water pump
I've changed my water pump out on my old 1987 90 hp Merc numerous times and would never think of doing it while the motor was in or over water.
I always leave my motor in neutral when changing the WP. The key is not to change the position of the lower unit gear shifter. Come to think of it, there is a type of splined bushing that sits on top of the lower unit, that connects the gear shifter. As it just sits on top of the lower unit (when the lower unit is removed), it could easily dropped overboard. Same thing goes for the water tube seal which connects the water tube to the top of the wp housing.
Not sure if your '03 would be any different than my old motor.
2005 Triton 220 LTS, 225 OptiMax