Which underwater detector for my conditions?

[Anonymous]

I would appreciate a few suggestions for a detector to locate coins with high silver content. The search area has sand on the beach about 1 meter deep max with granite rock underneath. When the tide goes out it exposes mud/clay on granite rock. The detector needs to be waterproof, as diving to 6 meters may be necessary. It is not a place where you find many pull tabs but does have junk from wrecked boats.
Thanks Indy