While you are scrolling through oceans and islands on the strategic map, your personal "GPS" is keeping track, stating coordinates and ocean number as you are traveling over the map:
That´s much more precise than ocean numbers in the corners, and you can even mark and save coordinates in order to revisit them later.
If you think that´s not sufficient, the Grepolis Report Converter might offer even more ocean numbers on the map than you wish for...