US definitions:
Over water is defined as flying more than 50 NM from a shoreline. If your aircraft is not over water equipped and certified you can not fly outside of this limit.
If your aircraft is overwater equipped and certified you can fly out side this limit but must not fly more than 1 hour flight time away from a suitable airport. This is an ETOPS limit.
If your aircraft is over water certified and ETOPS certified you can pretty much do anything.
Our company has special authorized extended overwater operations authorization by the FAA. For example we can extend the flight of our non-overwater aircraft to 100 or 162 NM from the shoreline in some areas, with restrictions. This may be what the MEL is referring to. Your MEL may be generic, some operators my have extended overwater authorization on the ATR72-500.
Take care.