Water Heaters - Dielectric coupling

Last weekend I ended up replacing my water heater. I still have some questions.
Went to Lowe's and bught their best 50 gal unit. The salesman also sold me two dialectric unions. This made sense and these looked like they actually separate the copper piping from the galvanized innards of the water heater. Except, when I got home, took all down the basement and wanted to start installation, the screw-in part of the union turned out to be too short, the female threads on top of the water heater are sitting in about a 1 inch-deep hole. I could screw that part in by hand, but could not put a wrench on it.
So, off I go the my local hardware store for different parts and advice. Here, they sold me these two galvanized nipples, about 3" long, with a plastic insert and said this is what I need.
I assempled it all with these nipples, but it still bugs me: the threaded copper coupling is directly connected to this "dialectric coupling".
How will this help preventing rusting of the galvanized part?
Also, in the many attempts of inserting and removing them, I noticed that the plastic was disfigured at on end on the hot side. Almost looked like when plastic is overheated. Any thoughts?

Use water heater flex connects they look like these, instead of the union.

http://doityourself.com/ori/200x200/6148175.jpg

Thanks Journeman.
Do I still need some dialectric connection between the copper flex and the galvanized inlet (and outlet) of the water heater?
Happy Holidays.

Forget the dielectric unions. The dielectric nipples you have are the best. The plastic lining, which rolls over the galv. at each end: the water never touches the steel, hence no corrosion can take place.

Yes forget the di-elec union.

Yes forget the di-elec union.
Hello again.
So, I went back to Lowe's and found the flex tube installation kit, including compression fittings and dielectric nipples. I see that the all female ends on the flex tubes have rubber washers. I guess this is there to seal off against the plastic insert of the dielectric nipple. This makes sense. So, my question: do I need to get similar rubber washers for the ID of the connections on top of the water heater, to keep the water from getting between the plastic liner and the body of the galvinized nipple?
Thanks for your help.
mike

You don't need anything but 2-3 flat wraps of teflon tape wrapped clockwise only on the threads of the dielectric nipples to connect those copper flex tubes with the brass fittings.
You don't need anything on the compression fitting ends but clean pipe to tighten them onto.
Just tighten everything as tight as you can (use backup wrenches on the compression fittings), and it will be fine.
Good luck!
Mike

Since the flex lines are made to break the contact between the two different metals, no di-elec union is needed, just tighten up as suggested.