If you spend a lot of time in fresh water you may be better off with a PuraSan http://www.raritaneng.com/products/waste_treatment/purasan.html instead of the ElectroScan (nee LectraSan)...it was designed to be used with toilets that use fresh water, but works equally well with salt water. It doesn't make chlorine, so it doesn't need salt...which eliminates the need for a brine tank. The so-called "tablet dispenser" doesn't actually dispense any chlorine tablets...Instead, water is siphoned through a cartridge (the "tablet dispenser)" mounted on a bulkhead that contains a solid block of hypochlorous acid, carrying enough away into the treatment tank to do the job. Price is a couple hundred less, amperage draw is lower because there's no electrode pack--which also means less maintenance...otherwise works pretty much the same way.

As for using the toilet without running the ElectroScan, LectraSan OR the PuraSan...yes, you can get away with that if NOTHING except urine goes into it...but if any TP (which means any time a woman uses the head unless she puts the TP in the trash) and/or solid waste is flushed, it must be run to prevent it creating a clog. The average liveaboard couple with a LS uses less than 20AH day...it would be even less with a PuraSan.

As for whether to install a y-valve to by-pass it...it would make more sense IMO to put in a 10-12 gal tank and y-valve that provides a choice of flushing into the ES or into the tank. That will make you legal everywhere...and in a crunch you CAN dump the tank.