You don't say how the suction into the bucket of antifreeze mix was set up, whether it was in addition to the main cooling water coming in from the outlet on the saildrive, or if it was instead of that main flow. If it was instead of the main flow from the sail drive and if it was just an open ended hose for sake of example, did you hold the end of the hose away from the bottom or sides of the bucket? If not, then the suction could have resulted in the hose end becoming sucked against the bottom or side of the bucket and thereby cutting off the flow of coolant with consequent overheat.
