Could anyone tell me this? Is the saltwater one much heavier than the freshwater one? and take a heavier line to bring out its flex? So will the saltwater wt 7 rod be much harder to cast all day long?

Thanks for any input.

I am not understanding your question, both rods are identical. Now lines are completely different.

Some manufactures have saltwater fly rod and freshwater ones as well. I just want to know if there is any difference between those rods.

Some have more than one stripping guide on them...

Which ones, because they will do both. They might have a couple of differences but usually are the same. IE: Fighting Butt, no fighting butt.

Depending on the manufacturer....The saltwater models could have a faster action.

corrosion-resistant materials?

In my humble opinion -

The saltwater specific rods should be designed for dealing with the conditions found most often in the salt. This means bucking a constant breeze, a corrosive environ and big, mean fish. If I were designing a saltwater flyrod, it would have slight differences from it's freshwater cousin in the same weight to account for these differences. However, that sort of thing costs money in research, development and manufacturing, so I don't know just how different, Really, that the rods will be. People like Sage and TFO just might do it. I bet if you shot them an email asking about it, they'd be glad to tell you.

Thanks, I think you are right about this.

That is most likely the best answer.

I don't know the "real" answer, but from what I seen, I think there are just some component differences. Primary, no fighting butt and a wood reel seat on the freshwater version, while the saltwater version will have an aluminum or composite reel seat, a fighting butt, and no hook keeper.

Some manufacturers will offer the same rod in either a 6 wt or 7 wt rod as either a freshwater or a saltwater versions. When I've compared them in the past, those were the only differences that I could find.

Rex

As Rex and Robin note, the big difference is in the components, with the saltwater rods having corrosion-resistant reel seats and guides and usually a fighting butt. Manufacturers may also talk about a "saltwater action" in their rods, which usually means they are fast action, tip flex rods that are stiffer in the butt sections to better handle the windy conditions you'll often find on the coast.

I don't have any of those saltwater fly rod with a fast action tip and stiffer butt section. So could anyone tell me if it is hard to cast with such stiff rod, and you need a perfect timing to cast it? and hard to get the perfect feeling? or I can ask you this way, are you willing to cast a light fly rod with a medium action all day long and really having fun? or have to work very hard to cast a stiffer saltwater one?

You shouldn't have to work harder to fight the wind but you darn sure need to work smarter. IMO, you need to have a good grasp of the double haul and how to make it work for you.
Unless you are blind casting a lot, you aren't likely to wear yourself out. It's mostly sight casting to spotted fish or likely looking potholes.

Double hauling is like any other aspect of the sport. How good you get depends a lot on how much practice time you are willing to invest. A medium action rod can double haul just like any other rod. You just need to give it some "lawn time" and you should be fine.

In the wind, you need to throw tight loops. Double hauling helps tighten the loops. So does a faster action rod. All things being equal (including your casting ability), a faster action rod will throw tight loops more easily than a medium action rod.

Just as with any activity, from carpentry to auto mechanics to saltwater fly fishing, it's easier to do a good job if you have the right tools for the conditions at hand.