I don't know how these things are laid out, but if they are like a car heater, it sounds to me like somebody needs to come up with a couple of valves to turn the water off at the pressure and return side of that core, and then a drain valve inside the loop.
If ya spring a leak, shut the water off on both ends and ramble on. At the end of the day, drain it to prevent crud from forming. Honestly, I'd leave it drained and closed util it got cold enough to need it.
Of course, being an old fart, if it every got cold enough to need a heater, I'd probably not be on the lake in the first place.