I do not know if the Kawasaki water cooled is quieter than the air cooled. I fixed a no start Scag with a few switches, removed jumper wires and replaced missing fuse holder. Then I moved on to new deck spindles, straightening bent deck, cutting out a bigger than 1 foot square section of broken deck metal and welding in a new patch, welding loose metal underneath, replacing missing bolts and on and on the list went. Went to mow and it looked horrible. How can that be as I measured and measured. I did not measure the new blades that were on the mower when I took it to repair. New blades a few inches longer and that mower cut like a dream. Quiet (for a 61" mower) and great quality of cut. That non-profit has a newer Scag with an air cooled engine. I guess I should try to do a test to see if the air cooled Scag is noisier than the water cooled Scag. The water cooled has only 1,000 hours and is maybe 16 years old. So much abuse and rigging/crummy repairs. I do not know how many hours I spent getting grease fittings to take grease. At least 2 full days. One full day cutting apart, pressing apart and welding back a parking brake actuator. All because of lack of proper greasing. Grease your fittings, even the ones with plugs that look like they do not get greased.
I too wondered if there is much difference between a Ferris, Scag, John Deere, Husqvarna, Ariens or whoever you consider to make a premium or heavy duty mower. Just keep it clean, painted, greased and oil changed.