Definitely not urban legend! Shore water connections left on when no one was aboard sank an average of 10 boats a year on Lake Lanier during the last decade I was there...I personally know of two very near misses--one sailboat owner who woke up in the middle of the night to use the head
and stepped out his v-berth into calf deep water...another who only took his dog for a run on a Sunday morning and returned after just an hour to find several inches of water covering his cabin sole
. And that's just on one inland lake!
Boat fresh water plumbing
isn't designed to withstand "shore water" pressure..Pressure reducers are an integral part of shore water connections...but there are only two types of pressure reducers--those that have failed and those that will...making it absolutely essential that shore water only be connected with someone is aboard and AWAKE to hear the FIRST gushes in the bilge
...'cuz the volume is more than 3 big bilge
pumps can keep up with.
It's not enough that water on the dock
just be turned off when you leave or turn in for the night...the hose should be disconnected from the boat.
The systems that only refill a water tank are a bit safer than those that install downstream of the water pump, but not enough safer to make it worth the risk to leave 'em connected.