Simon
Zantedeschia bulbs like a moist soil.
I use Z. aethiopica (arum lilly) as a marginal pond plant but they can
apparently be planted to a depth of 12 in of water. It has a rhizome
rather than a bulb.
I plant mine in pond baskets, so depth is fairly shallow - 4-5in. It
is a borderline hardy plant. At the moment, the leaves are about 5in
in length, sat in a pot on the cobble edge in my wildlife pond.
There are other Zantedeschias that grow in moist soil, however most of
those are frost tender.
I guess the question to ask, is what variety are you trying to grow?
Clifford
Bawtry, Doncaster, South Yorkshire