(Daryl J, a marine aquarist and hobbyist for the past 10 years and still actively involved in pursuing the knowledge of setting up a perfect enclosed saltwater tank system)
Using natural seawater for your saltwater fish tank is actually a viable option to be considered if you are staying near the coastal area and thus obtaining the water is not a hassle. What’s best is that you can always get it all year round 24 hours a day except on occasions when there is an environmental pollution affecting the cleanliness of the water. Put that aside, there are also other reasons on why using natural seawater has its advantages especially on the part when it comes to emergency situation when you need to quickly change the water such as the condition when water quality suddenly deteriorates and becomes unsuitable for the fish to live in. A fine example would be ammonia or nitrite jump in a newly setup saltwater tanks or it could be due to failure in the natural filter system sometimes caused by unexplained reasons.
Despite all that have been mentioned here, using natural seawater has its drawbacks. There is always a questionable issue related to its use because most aquarist found that natural seawater is not perfect to sustain biological marine life and keep them lively meaning that the water has to be constantly replaced. Try to imagine this scenario similar as using a second hand used water in which the quality will deteriorate fast. Although still much a subject of debate and discussion, most find that saltwater aquarium using this natural seawater often has issue with the fish showing signs of lethargy and uneasiness. Part of the problem is also believed to be deficiency in certain micro nutrients and chemical composition and that is why some hobbyist prefers to use prepared synthetic saltwater instead if they want to provide the best living condition for their pets to live in.
Natural seawater also increases the chance of introducing unwanted microorganisms and living sea creature unseen with the naked eyes to the reef system. Most often, despite treating and leaving the water in the dark for specific period of time to remove of these unwanted guests, somehow without valid explanation, they still manage to survive. Some people go to the extent of running the water through an ultraviolet UV light in order to sterilize the water but if you are spending your time doing all that, it will actually defeat the purpose on cost-saving using ocean seawater. Although a more direct approach involves treating the water with chemicals prior to use such as chlorine bleach and copper, somehow the microorganisms still has a way to survive the treatment ordeal. Thus, there is always a quarantine period here that needs to be taken into account and I would strongly caution against pouring the collected seawater directly into your saltwater aquarium without any form of safety precaution put in.
But despite this potential threat, there are actually some uses with the presence of microorganisms in the natural seawater. New aquariums which have just been established will not have the benefit of beneficial population of bacteria exist in the system. In other words, depending on where you collect your water, these natural seawater can actually used to jumpstart the tank biological cycle, very much similar like what you have with startup bacteria liquid culture which you can buy from pet shops. Finally after evaluating on all the facts stated above, it is to my opinion that unless cost is you main concern towards maintaining the fish tank, it would be best that synthetic saltwater are used instead to ensure that everything is working well without the risk for your pets.
Related saltwater fishkeeping topics: Live rocks for reef tank