Hi there,
Congratulations on your purchase of a Canon Ixus - I've just recently returning from trying out one of these in the Red Sea and got some lovely results.
With regards to weights, they do help a little to keep the housing still underwater. What happens is that the housing is a little buoyant which means it can wobble a little and a little weight will help it keep it steadier underwater - a little similar to ourselves really!
I didn't use any weights with my Canon Ixus and got some nice shots, so the choice is really yours, they do help a bit, but aren't completely necessary.
Happy shooting and don't forget our special photo trips for cameras like yours!
Best Fishes,
Maria