Timber slashes at the rusty chain and severs it.  With a crash and a large splash the chest hits the water and sinks to the bottom of the pool.

Dr A turns around at the sound of the splash only to watch the chest disappear into the water "ahbut I don't think that will satisfy your curiosity my young friend. What, if anything, is your Plan B?" 

OOC: Whilst i completely missed there was a pool, it works with Timbers act and then think personality haha

DM: How deep is the pool?

3 feet 

Kirinar laughs. "Well, that's one way to get the chest down from its chains. I wonder why it was held above the water, though...should we haul it out and see what's inside?"

Ransyl examines the water, “It seems like there might be something awaiting the person that gets in. That’s what I would do.”

Investigation: 3

Timber will look around, "Unfortunately, I don't think I'll be able to help lift it out, as I am not the strongest. In regards to a trap, Doc, do you have any ways to see if anything is hidden there?"

Doc, do you have any ways to see if anything is hidden there?"

"Ahbut that would be a no. And no I'm not going swimming in their either. You'll have to come up with a better Plan B than that young'un if you want to know what's in the box!" Dr A responds.

"Ahh, come on. It's not even chest deep. Get it?!!?" 

OOC: Timber is itching to go get it but with a (-1) strength, I don't think he could do it.

Quote from Zipnok >>

OOC: Timber is itching to go get it but with a (-1) strength, I don't think he could do it.

 Even with a 9 Str you can Push, Drag, or Lift 240lb.

The young man grabs a couple of the chains and pulls on them, planning to drag the chest out of the water.

Athletics: 18

With some effort, Kirinar is able to drag the heavy chest along the passage and get it out of the water and into the room with the dead wights.  

"OK, Keyleth, do your thing!" the fighter says, proud of himself for dragging the heavy chest all of the way back to the large room.

"Well how can I say no after all of that?" Keyleth says, pulling out her tools. "You'll save me if it eats me or something, right? Dr. A, do you think Mystra wants us to open this thing?"

She takes a few deep breaths, kneels down in front of the chest, and gets to work...

Thieves' Tools (assuming Guidance): 23

Dr. A, do you think Mystra wants us to open this thing?"

"Ahbut most definitely my son. All knowledge is recognised by the Mysterious Lady as ultimately being of some benefit!" Dr A says as he casts Guidance 

Receiving guidance from Dr A Keyleth feels confident in her ability to pick the lock.  She deftly slips the thieves tools into the lock and begins to work the mechanism.  But to her horror the demon skull bites down on the thieves’ tools and destroys them.  The chest remains locked.

Keyleth's hand recoils in surprise as the skull bites down on her pick.

"Well... I suppose it did warn us that it would devour all but the greatest thieves. And, well, I'd like to think I'm a pretty good thief, but I would never pretend to be one of the greatest. Considering the setting," she gestures vaguely around at the dungeon, "I suppose we could have taken worse than a broken lockpick."

"Shall we move on, or do you want to try bashing this thing open?"

OoC: Is the entire set of Thieves' Tools inoperative now, or just the one lockpick? PHB says the kit comes with a "set of lockpicks", but I'm not sure how that works, exactly.

"Anybody sporting a crowbar? I'll take a 'crack' at it." Kirinar states, deadpan, but grinning at his clever puns. =)

Quote from Esker >>

"

OoC: Is the entire set of Thieves' Tools inoperative now, or just the one lockpick? PHB says the kit comes with a "set of lockpicks", but I'm not sure how that works, exactly.

I'll say that some of the tools are wrecked.   You can still use them but you get disadvantage on the rolls because the set is incomplete and therefore you can't properly use the set anymore.