Belt's way too tight!
You can run it quite a bit looser than specified as the wrap on the pulley is what provides the drive. That little water pump requires VERY little power so the belt dust is probably because the pulley is out of alignment, a result of the bearings having failed. (my Yanmar belt is coming up on 1100 hours, and is still in good shape).
schedule is to a) replace the impeller every second year. Johnson Pumps have impellers that substitute for the "proprietary" Yanmar. (eg: the 3GM30F uses Johnson's 09-810B-1 kit for their F4 series. I buy for about $35.
b) Replace bearing and seal every four years. Most industrial type suppliers have the exact same bearings, and stainless steel
seals; my total cost $28 this year.
Take a look at the pump cover plate, if it has significant wear, at such low hours as yours, it is due to the bearings running out of true, due to that excessive belt tension. May have to replace, or make, a new one.
Take care to deflect the impeller blades in the correct direction as you assemble the pump and lube with generous coating of non-petroleum oil
Make sure the sea cock is open before firing it up and you should enjoy the next 2 years of trouble free use.