JB, JB, JB...
That's not entirely correct. One can (and I do successfully) take the 250 onto the wet sand and even into the shallow water. I have posted a couple of threads and posts about this. I was told by Brad at Garrett Tech Support that it would work and with the DD Coil, it would be even MORE stable and by golly, mine is!
Here is the first thread from hunting in the water, before the DD Coil:
http://metaldetectingforum.com/showthread.php?t=70584
And here is the thread where I included pic's from a second wet sand hunt and the quote of the email from Brad:
http://metaldetectingforum.com/showthread.php?p=836894#post836894
Granted, the 250 isn't going to work like a PI or a VLF with ground balance capabilities, but it WILL WORK in the wet sand. Of course I don't know where the OP is at so the sand may be way more mineralized than ours here. Bottom line, he should have a go at using it in the wet sand and see how it does for him.
As far as a cover, I hunt the beach 95% of the hunting I do, I carry 2 one gallon sized ziploc bags - 1 to cover the control box as shown earlier and 1 to put my headphones in - and a sandwich sized bag for my cel phone, all just in case of rain.
Good Luck and HH!