Put new cogged belts on - all are now noisy from idle up - the water/fuel pump belt is the noisiest. Sprayed belt dressing on - the noise stops for about 2 hours. The motor is a GenV 502 mpi.
Changed belts because I had the same problem last year - is this normal for this motor ?

There's several things that'll cause a belt to make noise.

1. Is it loose?
2. Is something binding causing it to slip?
3. Are the pullies aligned properly?
Just because Merc assembled it doesn't mean they can't install
things wrong. See story below.
4. Are all the water pump, alternator and power steering pump
brackets tight and not allowing them to vibrate?
5. Does the belt have the right 'V' angle?
If the angle is too narrow it won't make full contact, ride too
deep in the pully and slip. Just the opposite if it's too wide,
it'll ride at the top and slip or fly off at high speeds.

Hope you find your problem.

My wife had a Dodge Daytona Shelby Z. One day the serpentine belt went bad, it had around 60K miles on it. Installed a new one which promptly destroyed itself in two days. What I found was that the water pump was out of alignment and that the pully had been installed backwards at the factory. How the original belt lasted so long I'll never know.

Belts are all tight; belts are level with the top of the pulleys, so I believe they are the right v angle; made sure all the brackets were tight, still have belt squel from seawater/fuel pump belt.
Installed new impeller last year and bearing were fine, so I don't think it's the pump. Unless it could be too much strain on the belt to be on both the seawater pump and fuel pump. Thanks,
I appreciate all comments.

i had similiar problem once, was slight pitting on the pulleys. took them off sandblasted and repainted them and the problem stopped. just somethingto look at?