Island's water centre (6)
I believe the answer is:
lagoon
This might be a double definition.
'island's' is the first definition.
The definition and answer can be both natural objects as well as being singular nouns.
Perhaps they are linked in a way I don't understand?
'water centre' is the second definition.
'lagoon' can be an answer for 'water' (I've seen this before). I am not sure about the 'centre' bit.
Can you help me to learn more?
(Other definitions for lagoon that I've seen before include "Water protected by a coral reef" , "Salt-water lake, often blue" , "Atoll's enclosed water" , "Almost enclosed body of water" , "Atoll lake" .)