Hello All;
I have a 2006 Chaparral Signature 350 w/twin 496 Mags. I've made steady progress on many performance issues. I just solved a nagging problem with belt noise. Here is the run thru and the solution I found. I hope it can help some of you.

New belt - no improvement 57-865635Q01 - still squeaks and screams on acceleration.
I tried the water trick (dripping some onto the moving belt) to calm belt slip noise. It works for minutes and then the noise comes back. This is supposed to point to it not being bearing related.
I checked with a automotive stethoscope and found no smoking "Squeaky" smoking guns.
I checked for free spinning rollers, tensioner, water pump (internal and seawater) and alternator. All where fine but I did replace the Tensioner roller just in case. No joy.

I degreased the belt and all rollers, pulleys etc.. This helped but did not get me to 100% clean. Starboard engine is perfect and the Port was still making some noise.
The final move was to replace the Mercruiser belt with a Continental "Elite" Poly-V belt with Quite Channel technology. These belts are very new technology and claim to be 15dB more quite.

The matching belt I found to be 4060990 and as I learned buying one too short is that last 3 digits are the length. It fit perfectly and works perfectly. No noise!!!!!
The best parts is this is a premier automotive belt and its still 50% of cost of the Mercruiser part. Of course I'll hold the original and another new spare in case.

Take Care
Vince

That's good info. I've been fighting a nagging belt squeak (and a small amount of belt dust) for a couple of years on my 2004 496 Mag. I can remove my back seat and sit right in front of the engine with my nose to the camshaft and I still can't tell what's doing it. New belt, cleaned pulleys (no rust), tighten, loosen. Nothing helped. It would squeak once every revolution. One small squirt of amour all on the belt and it stops for a day. I'm replacing the seawater pump this spring with a new Hardin unit with the billet pulley and if that doesn't work I'll need to try your belt.

no offense towards you --- but I seen pics of your engine. Your pulley's has alot of rust on them. The rust acts like sand paper and abrades the belt (s) in a quick manner. Your new belt will get and act just like your old belt down the road. I would address the pulley's asap as well. Also make sure all pulley's spin very freely again.