Before I give you the script remember that jumping will be messed up because of the eddittoggle. Also remember that this can only be used offline or in coop edit mode. And if used in coop edit mode then other people will not see the water like you do because assault cube only shows changes in local waterlevel to you unless you send them the map.
[SELECT ALL] Code:
alias waterlevelup [
waterlevel -10
[waterlevelup1]]
alias waterlevelup1 [
[edittoggle]
alias test 1
alias newwater (+ $waterlevel $test)
waterlevel $newwater
[edittoggle]
sleep 900 [waterlevelup1]]