Tracey,
The red plug that came with it will NOT hold the water back, it's way too flimsy and can't seal properly, it's a rodent plug which is really not needed since they cannot scale the wall of the pool.
If you have regular winter temps like us here in canada, I would reccomend dropping the water level to below the aqualum. If this is something you really do not want to do, then you will have to source a rubber stopper (like you used in science cl***) to plug the hole successfully (from the pool side).
Putting in a burnt out lightbulb will not help since it allows water past it and down the angled input line and just stops water from coming out the back where the electrical connector clips on, which can't have water in it because expansion will crack that all to pieces.

Dennis,
I may have to take you up on getting one of those burnt out Aqualum bulbs off you, I've been waiting patiently for mine to die so i could "experiment" with some custom LED lighting that I plan to insert into the original fixture, however, mine just won't die - unlike the norm!