I would agree that storeing more water might be better than removing the salt from sea water.
Reverse osmosis is a well established technology but requires considerable human effort (in the case of lifeboat type units) or electricity in the case of larger units.
Futhermore anything mechanical is liable to failure, therefore a spare would be sensible which adds to the already substantial cost.
I believe that coastal seawater can be too dirty for optimum use in reverse osmosis plant, many yachts are fitted with such, and the advice is generally only to use it well out at sea, not in port or near a coast.

If you have a garden that requires watering, consider installing a large tank, kept filled from the mains water supply by use of a float valve.
Use water from this tank to water your garden by means of a small pump, this will ensure that the tank only contains fresh water, that can be used for household purposes.
Your existing 60 gallons would last a long time if reserved only for drinking, use the tank water for washing, cooking and laundry.