In Sadistic's guide to getting Emblems without the Mystic Melody, he says that the Melody is needed for the Treasure Scope. It's not.
Simply glide at the gap above the metal crates and drill dive once you're really close to them. you should destroy the top crate. From there, it's easy to get in.
Unfortunately, this doesn't allow any more Emblems to be gotten, although it certainly makes Dry Lagoon 5 easier.
Oh, and if this is old, sorry.