There are many possible ways. One is not to bother with coordiantes but use the locations of objects already on the map. Turn Object Ids on in the mission editor, find the ID number of bush or something and then do a
getPos (object XXXX)
where XXXX is the ID number.
One method I use a lot is to find actual coordinates is to create a repeating radio trigger that has the following On Activationion code:
hint format ["%1",getPos player]
Then I go into the mission, move to the location I need the coordinates for and activate the radio trigger.
If you want coordinates that are in the sea then just put a boat or a game logic there, give it a name and replace player in the hint above with the object's name.
This will give you the x,y,z coordinates in that order. X is measured west to east (so x=0 is the western edge of the map). Y is measured south to north and z is measured vertically from ground level. For future reference angles are measured clockwise starting with North as zero.