Running in salt water will introduce it to a level of corrosion it has never seen in lakes and rivers. Salt is corrosive to most metals. You will want to make sure that your motor has a sacrificial zinc anode on it to keep electrolysis from eating the aluminum of the motor. You should also run/flush it with fresh water as soon as possible and clean all salt residue off of the outside of the engine as well. IF you are careful with these maintenance chores, you can run it in salt water with little to no effect. If you will be leaving it sit in salt water, start a bank account for yearly replacement of cooling system parts, at the least. If it is an inboard engine, your exhaust manifolds and/or heat exchangers will be a very pricey repair. Always remember; A boat is a hole in the ocean into which much money will disappear. Be prepared for the expenses and you can still enjoy it.