Let me guess. You're not from the United States, right? Or at least are native of another country before moving here. So some place like Mexico, or the Phillipines, etc... right? Anytime someone says they need to go "20 ft. deep" ("3 to 5 meters, etc...), this is the giveaway they're from some such country where those lores and legends abound. And sure, they're all iron-clad true though , no doubt.
Now that you've clarified you're after caches (and not individual coins, relics, jewelry, nuggets, etc..), we can start to get someplace. You're going to want a 2-box unit. Like a Whites TM 808. HOWEVER, let me tell you ahead of time: The ONLY objects that can be detected down that deep, even with a 2-box unit, are going to have to be the size of a volkswagon!
So how big is the objects you're thinking you're going to find?
And what makes you think this/these objects are, of necessity, that deep anyhow ? If someone intended to come back and retrieve the object at a later date, then why oh why oh why would they go to the trouble of burying that deep? A mere 1 ft. deep versus 20 ft. deep, are EQUALLY invisible from the surface. Right?