The only question left really is are these designed to be left o. For long periods of time
. "This heater is ideal for industrial applications where freeze protection or temperature maintenance is required, even in the places where space is limited, and using in a wide range of different fields and applications such as farm houses, cattle farms, construction sites, factories, industrial sites, military units, restaurants, outdoor events, and many other places requiring hot water."
If it's designed for frost protection, it must be designed for long term submerged use