I've conquered one. I was fortunate to have some good armor enchantments suited for the job, so in retrospect it didn't seem that bad.
Ideally you will want boots with Depth Strider, and a helmet with Aqua Affinity and Respiration. With a high Respiration enchantment, honestly, water breathing potions were almost not worth it. Also, if you take milk to get rid of the mining fatigue (and you should - I would say 5 buckets minimum), you will cancel all the other potion effects as well. You can place a torch on a wall, it will create an air bubble for a fraction of a second. Placing a ladder on a wall makes a permanent air pocket for you. Combine that with Respiration, and I really didn't use water breathing much.
I would go in from the top and try to find the Elder up there first. I used a strength potion to make it easier to take him down, but they're really not all that bad once you find them. Then the other two are supposed to be in the wings of the lower structure. These can be harder to find.
After I got those 3 Elders out, I was lucky enough to find 2 sponge rooms. They were instrumental in drying out the interior of the structure. I walled off sections to work a small amount at a time.
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Just started a world on this server. Very helpful to have all of the coordinates. I'll make sure to post anything that I find that isn't already listed.
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This really helps a lot. Thanks guys