Minecraft Java Edition – How to Prevent Dying While Drowning

If you don't mind "cheating", the easiest way would most likely be using MCEdit or WorldEdit. If this building is massive like you say, it would take a very, very long time to move it above ground using pistons. If you really wanted to do this, though, you would put a piston under a block, activate it with a lever, place a block in the new empty space, and repeat. Unfortunately, since you are supposedly underwater, this would be very difficult.

However, I have done testing, and if you were to push a column of blocks up using a piston and one of them is a water source block, the water simply disappears.

Edit: Now that I see you're an admin on a Minecraft server, I'll give you WorldEdit instructions. To use WorldEdit, first install it on your server. Then use the //wand command to give yourself a Wooden Axe. Use this to left-click on the bottom-leftmost block of your building, and right-click on the top-rightmost block. This encloses your building in a "selection". From there, you can //copy and //paste it wherever you please.

If I meet Quicksand on any map, it mostly appears like

So if you stepped in it you can easily dig the 2 neighboring blocks in a depth of 2, kinda like this.

And the you can easily walk off the quicksand the direction you dug.

The most important part is the depth of two blocks to dig, if you just dig one block, it's too high to walk off.