Wow...a lot of worry warts....
I'm in the salvage
business and in the last 11 years I've raised maybe 250 boats. Not one of them was from a "burst" fresh water line. My own boat has has the hot water line "split" no less than 10 times in the last 3 years because of the way the PO had it set up and the "spare part" hose he had on board to fix it...it was obviously junk hose. The "baby" bilge pump
kept up with the split for more than 6-8 hours so I never had my high water alarm
come on...the only way I knew was the hot water seemed less than it should be.
Your boat actually takes awhile to sink from a "burst" hose.....especially if you have your boat plugged in, the charger
on and a decent "little" bilge pump
Many....many folks use city water on an every day use and just turn it off on the dock when they leave the boat for more than a few hours.
Some people have "water meters" attached to their hoses that can dial in a "set number of gallons" before they shut off. I have one and use it sometimes...works great till a really cold night freezes and cracks it's internals...
So yes...city water can sink your boat...I always thought the same and feared the same. Till I got all growed up and realized hoses rarely fail in the "full mode" unless your preventative maintenance
really stinks...and even if they do..your boat if it has even the minimal bilge pumping capacity will not sink unless you leave the boat for more than 8-10 hours or so.
But I do agree that using your tanks and pump(s) is a good idea to keep the system fresh and know that it works fine. My marina just lost
city water for a day and the liveaboards without full tanks and/or a working pump were really sorry...Glad I know that both methods have their plusses.