Depending on the engine and the garden hose flow, most larger engines will pump more than the garden hose can provide. So if you leave sea cock open, it will draw in a mix of fresh and sea water. That's why I close the sea cock and just stuff the hose in the open strainer. Then it pulls in a mix of fresh water and air, and no sea water. If system is sealed up aside from the fresh water hose, it will pull a partial vacuum on the strainer, but that won't hurt anything. And yes, if hose collapses, it is only collapsing partially, water is still flowing. It does not need much water to do a flush at idle.