Unless the gearcase water pickups are running perilously close to the surface of the water where they can start sucking air instead of water (like a bass boat/jack plate situation), as long as the water pump is in decent shape the engine will run in the temperature parameters designed for it, and the alarm will come on if there is overheating. You'd probably hear the alarm before you'd notice the required pressure drop on the type of boating you do in your center console.