You can use this motor in salt water. You need to do a few extra steps after each use - nothing major:
1. ALWAYS, always, always, flush the motor with fresh water after you run it in the salt. Use "muffs" which fit around the lower unit at your water intake. You shoudl connect the water, muffs and start the motor and let it sit at idle for at least 10-15 minutes. This will prevent mineral (salt, etc.) deposits from forming on the head, intakes and anywhere else the salt water touches;
2. Scrub the outside of the motor with a bio friendly detergent (like Simple Green) and a soft scrub brush - pay careful attention to any metal (prop, lower unit, etc) and rinse completely;
3. Open the cowling and dry out motor - use a little CRC spray on any exposed metal
Also, rinse off your trailer - salt water is pretty corrosive and you do not want it left to dry on any metal.
HAVE FUN