I think this idea could actually work. I'm thinking similar to buried treasure, where gralats (100-499) and a few admin hats are hidden under water in a particular area (it would be a pretty large area, like buried chests), and you use your flippers to find it. If you dive in a spot that's close to one of the hidden goodies, you get the same "you're getting warmer message", and if you're under water while on the exact spot the item is hidden, you'd get the yellow exclamation point, and you'd use the flippers again to pull up the item. There'd be no dots (in fact, regular loot in the area should be disabled during the course of the event). The event could be called "Sunken Treasure".

Although, perhaps to separate it more from buried chests, instead of gralats and/or hats (or even in addition to), maybe there could be event-specific loot (which could include loot items that become furniture). Specific loot items you can only find during these events. These would be loot items you'd really want to show off in display cases

I actually really want this to happen now. I really think it would work nicely.