I know this thread is a little old but yes, you do need the High Seas xpac to decode the map. I'm not sure if someone without the xpac could dig up the map after it was decoded. I've done one and it was horrible, spawned two renegade changelings (this being in Ilshnar) that were like paragon Irks... just about the worst thing I've ever tried to solo. After calling in help to kill the guardians there was nothing remarkable in the chest to show for it, just 35k 7 essences and a minor artifact from the same loot table as the level 6 maps. Oh, and more junk. I think the only special it that drops from the level 7s is the master skeleton key, which is a single use item. I know that one map isn't enough to accurately gauge the level 7s, but after the first I dont think I'll be doing another one.
