Here in FL in the stores and wholesale houses I worked at, we only used natural water so when done properly, it's okay to do. To be a bit more specific, you have to be careful where you get the water from. It's best to get it on an incoming tide not outgoing. If your water typically carries a lot of pollution from trash or business, I would suggest not using any water and do the artificial. If the area is not pristine where you collect the water or is filled with overwhelming algae, I'd be hesitant to collect there. You will be bringing in more than just water ( in the form of free swimming diseases and parasites) so if you are not using any type of sterilization or filtering for these things, I would not do natural water directly from the ocean. Which brings me to never get water from a bay but from the ocean itself. Offshore is better than inshore and when a current is running is better than slack tides. If you do have the opportunity to collect good water, I'd collect enough to have extra in a container to use at a later time. Keep it aerated for best quality. Lastly, you always want to check salinity prior to using the water. After times of little rain, the salinity can be higher than normal so you need to know this before you use the water.
It sounds like a lot but it's pretty simple.
You just need to be aware that water is not always good water even tho it comes from the ocean.
Hope this helps.