Marine scientist here, don't take ocean water from random spots, because you can't be sure if you are going to get only salt, because sea water isn't only salts, so just buy it from a store or if you want, go through the trouble of not only containing sea water, but also checking it's pH, salinity and let's not forget to measure the concentration of many different chemical components in the water to see if those measures are on the limit of being healthy. That's so much more than 4 hours. Edit: woah, damn, didn't know I would blow up like that, especially with replies, thanks. One last thing, if you are disagreeing with me, it's okay, but that doesn't mean everything I say is wrong, so if you want to reply, please reply with a scientific argument, not just blinded disagreement, thank you.