I have calla lilies growing alongside my pond but not in the pond. According to Allison Beck's Water Garden Plants for Canada the variety Zantedeschia Aethiopica can be planted in water up to 12" deep. They are a Zone 8 plant so must be dug up or brought in for winter. The species I mentioned can survive winter in Zone 8 if it is completely submerged in the pond.

I have them in mine - a couple different varieties. I do pull them and let them dry out and keep them dormant over winter. Then in spring, once they leaf out I put them back in the pond - I just put them right in the rocks I have at the edge of the pond.

You probably can keep them year round in Z9.

I've tried twice.....once in like 2-3" of water and once flush with surface...lasted about 1-2 weeks

I put it on the list of plants I kill

The big white callas Zantedeschia aethiopica can be grown in water and there is a native bog calla Calla palustris too that is much smaller and white but the colored hybrids can't stay in full time they will eventually rot.

Well, I am in zone 4A (eastern Canada). I have 4 calla (water arum). 3 grow loose rooted in rocks (they just floated to a spot and rooted on their own) and one I have potted in water about an inch over the crown. They all come equally well. I also leave them right in the pond over winter and they are fine come spring. Just my experience, could be different for others.

I bought one at lowe's along with several other plants and it has been blooming now for about three weeks. So even if it dies out it was only $6-7 and I am enjoying it alot.