Is there a way to raise water by 3 layers/blocks every 3 minutes? So I'm working on this map where water will rise 3 blocks every 3 minutes or 1block/minute. Is it possible??? If yes can you please help out?
i've never used commands, but my thought would be a command to change all air blocks in an area on one y level to water, then a timer to wait however long, then a next command block to change the next y level up to water, and so on (my mind has trouble with minecraft using y for height, its z in the 3d design/printing world)
For a small area that you know is loaded what webrosc said is easy.
fill <x1> <y1> <z1> <x2> <y2> <z2> minecraft:water replace minecraft:air
but you can only do a volume of 32767 blocks in one command, so an area of 104x104 for 3 blocks high or 181x181 if you do 1 layer at a time.
It would be relatively easy to increase the area a bit by adding more command blocks.
There's still the problem of whether all the chunks are loaded, if you're using 1.14 or later you should be able to use the forceload command to keep a whole area loaded but you need to consider lag of course, and I can't find info on how large an area can be forceloaded.
If you don't mind having some or most of the water under the surface be flowing you can cheat a bit by centering the x and z coordinates on the players position, using whatever the desired water height currently is as the y coordinate.
execute at @p run fill ~-50 <y> ~-50 ~50 <y> ~50 minecraft:water replace minecraft:air
That way the command block would constantly be adjusting the water level around the player as they moved around. If they moved fast enough they might be able to outrun the water and find themselves in a dry area with water above them that hasn't had time to reach ground level yet. That could be fixed by chaining command blocks so you added water at say, every 10 layers up to the desired height.
I see people doing fantastically complicated things with scoreboards so I'm sure it's easy to increase a variable by 3 each time but it might as well be black magic as far as I'm concerned.
What I personally would try in order to cut down on the number of command blocks needed would be to have an invulnerable named mob floating in the water and use coordinates relative to their position.
execute at @e[name=George] run fill <x1> ~1 <z1> <x2> ~3 <z2> minecraft:water replace minecraft:air
Or the height based on their position and the X and Z based on the players position, though I don't really know how to mix and match like that.
If you have a naturally generated world with caves you might need to double up on the command blocks to replace cave air with water as well as ordinary air (unless you wanted caves to be safe spaces.)
Another problem/feature with caves would be if you're only adding water at the top around the player then if they move to a new area then caves wouldn't be filled with water.