Natural sea water vs instant sea water

I've had freshwater aquarium for years with no problems. I'm ready to
switch to salt water. I purchased the necessary equipment and
supplies. Ocean salt in bags, Wet/Dry filter, skimmer, sand (store
bought)hydrometer, hi ph test kits and powerheads. I will be using a
55 gallon tank. I live close to the ocean. Is there any problems if I
go ahead and get ocean water VS purchasing sea salt and do my mix. How
about water changes. Can I use ocean water? How about sand and rocks
from the ocean?

Thanks
Rudy