Why cant you submerge it, its intended to heat water.
He wants to heat the water at all levels without submurging the heater totally.
Ok I've had a think, you need two floats connected together with a connecting bar, fasten the heater to the connecting bar, if the floats are round, the connecting bar must be round. and the heater attached with a ring, then if the floats turn the heater will remain hanging down.
Just put a stop each side of the ring to keep the heater away from the floats.