I would never use ocean water....if your LFS
then they are just mixing it themselfs and charging you for the salt and there time, its rather simple to mix though.
I got a bucket of salt for 69.99 and it makes 160gal of water, so I just use a hydrometer to test the SG
and im good to go, throw a heater and power head in the bucket over night and no problems.
When you first fill the tank dont assume you can just dump the whole bag of salt in the tank and be done, test the SG
before you add to much.