Well, finally I finished my function to set the water level for whatever value that you want.
Here I show you how to use this new feature:
$MyADT = new ADT();
$MyADT->ADT_Open("../../maps/Azeroth_32_48 - copia.adt");
$MyADT->ADT_HeaderInfo();
$MyADT->ADT_MH2O_info();
$MyADT->ADT_MH2O_Set_Water_level(15); // This will sum 15 to the actual water level
And some screenshots.