With a bit of time and effort, you take turns diving, hauling and keeping an eye on the lake. You manage to drag the entire hoard, down to the coin, out from the depths. Counting it up you find yourself in possession of:
1x Iron Coffer (no lock)
1x Full Plate (finely crafted, and bearing a coat of arms you do not recognize immediately implying that it is indeed quite old)
1x Glowing Purple Bauble on a Chain (this is originally a Periphat of Wisdom +2, however one problem with this campaign is the treasure assumes a 'typical' adventuring party and that's no fun. So, I'm going to try "flexible treasure" : this is a necklace of "Base Stat +2" where whomever takes it chooses the base stat it modifies and it will become that forever)
1x Magic Weapon (same as above, originally this was a mace but I don't think any of you use a mace. Decide who will take it, and I will make it into a weapon suitable for them).
1x Horn of Blasting (see below)
A gold necklace (800gp value), eleven gems (30gp each), 8000 sp, 2300 gp, 130 pp.
OOC:
Horn of Blasting
You can use an action to speak the horn's command word and then blow the horn, which emits a thunderous blast in a 30-foot cone that is audible 600 feet away. Each creature in the cone must make a DC 15 Constitution saving throw. On a failed save, a creature takes 5d6 thunder damage and is deafened for 1 minute. On a successful save, a creature takes half as much damage and isn't deafened. Creatures and objects made of glass or crystal have disadvantage on the saving throw and take 10d6 thunder damage instead of 5d6.
Each use of the horn's magic has a 20 percent chance of causing the horn to explode. The explosion deals 10d6 fire damage to the blower and destroys the horn.