There's osmosis. Well okay, water flows naturally from low salt concentration to a high concentration, causing a gold fish to die from shrinking of cells when placed in salt water and a clown fish to die from swelling cells in a fresh water environment. If frogs have the ability to suppress this osmosis thing, we can probably see adult frogs thriving in salt water. Upon searching the internet with the keywords "Salt Water Frog", I found links to Q&A sites. Most of these sites refer to two species: the Florida Leopard Frog and the Crab-Eating Frog.
The earliest known fishes happen to be fresh water inhabitants. These are the ostracoderms, which appeared in the Cambrian Period, about 510 million years ago . And this suggest an early fish to amphibian transition. In fact, diversification of amphibians happened earlier than do large sharks, ratfishes, and hagfish .
I guess this mean that egg and juveniles (young fish or amphibian larvae) early in evolution are entirely freshwater dwellers. After all, this make sense, some marine fishes migrate to less salty water or freshwater to breed. Examples are bull sharks and salmons. It seems to me that marine fishes that return to fresh water to breed may have rooted from the evolutionary lines of the earliest fishes or they may have changed very little at all. (Note that I made some speculations here)
This is bad news for me. No salt water breeding frog, that's it... I can't argue with fossil records. But wait, what if all freshwater sources become polluted or toxic to support breeding of amphibians? Then frogs can probably evolve to adapt to breed in salt water given no other choice... There it is, a marine frog!