How do i make a switch that lowers or raises water levels in my dungeon?

i have no idea how to  do it 

There are probably far more efficient and less time consuming methods, but the only way I can think of right now to get that effect is to make the map then copy it, editing the water level to be the other level(s) you want on the copies, and then make the switch actually transfer the player between the maps. This method would require any chests to use regular switches with the corresponding chests on each map set to keep them in the same state rather than using the self switches which would only effect the one on the map it was opened on.

If you're happy to use a script, Tsukihime has one that would do exactly what you want here: http://www.himeworks.com/2013/02/17/tile-swap/

At the bottom of the page I've linked to, there is a link to Tsukihime's tutorial, which uses the example of water appearing and disappearing - so you could see how it could be implemented in your game.  You would put the appropriate script call on the lever, or whatever mechanism you are using to raise/lower the water.

This is a very quick and easy way to achieve what you are looking for, and the script can be used for all sorts of other situations.

Well, I did say:

:p