What's the best way to find all (or most of) the water in an area in a Minecraft world (to be able to remove it)? Especially water in caves is a problem.
Since a big part of Minecraft is finding resources, the game won't make it too easy for you. You will have to go through the caves and search for it.
But to find water in particular, you can make your task a bit easier by turning subtitles on. Flowing water produces sound and if you have subtitles on, you not only see it clearly no matter how far away or quiet it is, but you also get an indicator if it's to the left or to the right of you.
As soon as you see the subtitle, you can dig into the direction it shows you.
The downside of this is that you can't find standing water that way, but those are mostly just single blocks, apart from very rare water pockets with multiple water sources. Both are only a small part of the water in the world, so they shouldn't make a big difference for your farm. If they do, you would just have to search more.
This was about an earlier version of the question. Apparently the asker didn't mean this, but worded it poorly.
= The manual way =
You can let sand fall into the water to split it into columns (7x7 should be good) and then drain it with sponges.
It should probably be the easiest to go into the water, place one sponge on the sand wall, break it, place the next one lower down etc. while you go down with the water. Then you use an ender pearl, elytra or pillaring to get back up and repeat it with the next column.
Alternatively you could also drain the top, then break the top of the sand walls, then drain more, etc., but getting the sponges back would be more difficult that way.
At the end you can break the sand either with a good shovel (instamining) or by breaking the lowest block and quickly placing a torch in its place.
Here is a video about it.
= The automatic way =
Since you're apparently a person that likes farms, this might be more your thing: You can build a flying machine and item dispensing system to automatically fill everything with sand. This works best in a rectangular area.
Since it would be quite difficult to explain the whole flying machine in text and pictures, I'll just link a video: