electric toilets are always noisier than toilets that use pressurized fresh water because sea water toilets have intake pumps, which can be even noisier than most macerating discharge pump/motors.
The intake pump
on the sea water version of the Raritan SeaEra pump which is a bit quieter than most...but prob'ly not as quiet as you want. Raritan originally offered the SeEra as either a sea water or fresh water version, but they separated 'em and renamed the fresh water version the SeaEra QC...at a price
that's not that much lower than the Elegance.
So your best choice if you want a toilet that's really quiet, is the Raritan Marine
Elegance...which is the top rated macerating electric toilet and not that much more expensive than a Quiet Flush or the SeaEra QC if you know where to shop...and I do. This online retailer has it for $750 INCLUDING the $150 optional Smart Flush panel. Boatersland Marine Elegance
(note that each of the boxes on that page is a menu).
The least expensive solution would be a SeaEra "conversion" which replaces everything but the bowl, seat and lid...but it's a sea water toilet. Defender has it for the best price Raritan SeaEra "conversion" at Defender