Immersive Ink

Yes the top of pool is a few feet above the floor of the room. So damaging the rim would allow some of the water to flow out of the pool and into the room. Assuming someone retrieved the non-coin objects then everything can be given a once over. It's difficult to tell if the rings are magic or not, perhaps Alrik can tell?

"I will wear these scars with honor. My ancestors would be proud of what we accomplished today," says Urist. "This Underhall was once a happy place full of feasting, drinking, dancing, singing. It's going to take us a while to salvage the treasure, so let's make ourselves at home." He borrows a torch and has a look around the room. Can the doors be barred from this side? Are there any other exits? Is there any intact furniture, maybe a table and chairs so we can sit and count the treasure? Does he see any artwork, sculpture, or vestiges of the ancient builders?

"Let me take a look at those rings," Alrik says. He examines them for any markings or signs of enchantment.

While Alrik examines the rings, Rhegaer looks for the lowest part of the fountain wall above the floor and begins working at taking it down to lower the water level.

"Wait, Rhegaer!" interrupts Urist. "Don't just topple it like a pile of blocks! We need to remove the stones in the right order, so we can put them back together when we're done. Let's start with this one first, uff da, that's right, set it down over here." The Dwarf considers the old fountain to be a historical artifact, to be treated with care.

Urist gathers his wits and craft knowledge and looks around the room. The doors can be barred using the old benches as wedges. That should hold well. The pool stones can be carefully dismantled the war released in any direction he chooses. Sadly whatever artwork once adorned the walls is long gone. Centuries of fires, battles, reek and neglect have created the lair that housed this young dragon.

With some clever work with a pole or staff the larger items can be pushed close enough to the surface to retrieve with a long arm.

Rings, horn, scroll cases they are brought up and Alrik gives them the once over.

Rings no hint that they are magical at all, but they appear to be good condition and worth a fair amount.
The scroll cases are bone and sealed with wax.
The horn, when Alrike touches he knows it is magical right away. There are fine letters carved around the outside bell written in the script of Anicent Elwhen

"When force is need sound deeply this horn."

Alrik holds out the horn, telling his companions what it says.

"This horn is for one of you, methinks. I do not believe the rings are magical, but will fetch a pretty copper nonetheless."

He breaks the wax seal on the scroll tubes and examines the contents.

Phadian looks on with interest. He looks pleased that Urist is up and about.

Urist and Squishy splash in the fountain recovering the treasure. "There must be HUNDREDS of pounds of gold here; what are we going to do with it all?" he ponders. Even the thought of dumping less-valuable copper and silver coin is heartbreaking to him. "I'd be happy to stand guard here if someone wants to go fetch the mule?"

Phadian

"We can bring Shoul, Rik, and Mule here and get things organized for travel. We may have to make some choices about what we will carry out and hide the rest for later retrieval. I do ponder if there is more we need to do here. Have we accomplished our mission."