If I were to go to a beach and get salt water, how long could I store it
for and how should it be stored?

I can probably get around 800 litres in one trip (2 1/2 hours each way)
but the water would then be in 200 litre (44gallon) drums with only a
small opening in the top. It's really not practical to go that far for
just one drum of water and hubby's ute is not comfy and airconditioned
and it's summer here.

Would the water go off?