Just a cheerful reminder that almost half the water we put into our system is lost along the way.
Also, I live at a rural property with rain water tanks, and we have a reasonably small tank for the size of our house. We haven’t come close to running out of water this summer, while we have had to get a tanker other years. It’s actually been a reasonable summer in terms of rainfall.
Seawater is a maintenance nightmare, it’s incredibly corrosive to almost any metal, so any pipework would need to be either plastic or concrete. It will also kill plants, so can’t be used for watering lawns etc, so this would only be useful for firefighting purposes.
Ships will often use sea water for jobs like flushing toilets, but I can’t imagine that being worthwhile running a second water main for.