9.9 4-stroke Yamaha outboard acting strange

My 9.9 4-stroke Yamaha outboard started up fine and ran fine yesterday on the first day of use this year. Today, started up fine but when I accelerate the
waterstream dribbles away. Doesn't happen in neutral or reverse, just when accelerating forward. Any ideas? Impeller blade maybe?

[boats.net]

I would replace water pump impeller to make sure it's getting enough water.