I am installing a new Marathon water heater next week. The cold/hot connections coming out of the heater from the factory are brass (or bronze?). Do I need dielectric fittings or just connect up to copper?
Not needed. They are used when connecting copper to galvanized pipe.
The inlet/outlet on my (now 10+ year old) State heater are steel. The original installer said dielectric fittings weren't needed, but the connections began to corrode over the next few years. I finally went back and installed them myself, plus a fat copper ground jumper between the copper piping.
That reminds me that it's probably time to think about replacing the anode rod again.
Don't most HW tank heaters have some sort of coated steel tank? I'd think that, if you have copper piping, you'd need some sort of break in there somewhere (maybe included by the mfr in the OP's example with brass fittings)
"They are used when connecting copper to galvanized pipe."
Water heaters are normally a glass lined steel tank.
The dielectric fitting may be built into the inlet and outlet of the tank.
Check the instructions.
If the outlets are already brass you should not need another dielectric fitting to connect copper pipes.
It would ultimately depend upon your local code.
Technically speaking, dielectric fittings are only required when connecting copper to a ferrous metal pipe appliance therefore if the connections on your water heater are copper the dielectric fittings would not be required,,,BUT..In some jurisdictions they require the dielectric fittings on all water heaters. Its a case of the code is lagging behind the state of the art technology, but you can't fight city hall.
Marathon's claim to fame is non-metallic tanks. No anode, no dielectrics.
As noted, if you had galvanised piping then connections would require galvanic break.