I don't think you can ever 100% guarantee safely using natural sea water. There is always the risk that you're going to bring something into the tank, whether it be a parasite, bacterial infection, algae, or outside contaminants such as gas, oil, or other chemicals. With that said there are a handful of reefers on this site that do use it, and have had success. To my knowledge one of them takes his boat out to get away from the shoreline to collect his water. As far as live rock and inverts please check with your local and state laws before collecting anything wild. In alot of places it is heavily regulated or even straight out not allowed to collect anything wild from the beaches or tide pools.