Mine are growing as a marginal in my pond in around 4" of water. They're left in all year round.

Depends which variety - Zantedeschia aethiopica 'Crowborough' (plain green leaves, creamy white flowers, can be grown in water or in the ground) is hardy outside, but its the only one that is. All the coloured flower forms, and any with spotted leaves, are okay outside in summer, but need to be brought in for winter.