There are more similarities between saltwater and freshwater fishing than differences. Both big and little fish can be caught in both types of water. You need bait for both and can use either live or artificial bait. You can go out in a boat in both salt water and fresh water and you can use a rod and reel along the beach shoreline or in a fresh water lake or river, so just what is the difference?
One of the main differences is the size of the gear used. The saltwater angler will usually use a reel that is much bigger than that of the freshwater angler. He is all set to catch that really giant fish. His pole will also be much stouter - just in case he hooks something the size of a whale!
Saltwater fishing allows the angler to fish in much deeper water. Even though some lakes are deep, they are usually not as deep as the ocean. So when you fish out in the ocean you will need not only a greater length of line to fish deep, but a stronger one.
Another difference is in the kind of boat used. A small boat is sufficient for freshwater fishing, but to go out in the ocean, something sturdier is required. You will certainly need two engines in case of malfunction. The ocean is a big place to break down in. You will need extra safety gear, too.
Technique is another area where the two differ. The lake angler will use a trolling motor and go around the perimeters of the lake and his casting will be in that area, via, the shoreline. Unless he is based on land in which case he will cast as far from the shore as he can. But generally speaking the freshwater angler finds the fish first, then tries to catch them
The saltwater angler on the other hand, concentrates on the opposite way by trying to bring the fish to him. He does this by dragging his lures along at the back of the boat in the hopes that a fish will be there. Of course some may have a sonar fish-finder of some kind aboard - not very fair to the fish when you come to think of it, but more fun for the angler.
Last but not least is the type of fish caught in saltwater versus freshwater fishing. Not only are salties bigger fish (in most cases), but they are different fish. Although some species like salmon and mullet migrate from their normal habitat for spawning purposes, most fish are classified as either freshwater or saltwater. Marlin and swordfish for instance will never be found in freshwater.