Ultimately, this deep sea is a source of fresh water (3)
I believe the answer is:
spa
'source of fresh water' is the definition.
'spa' can be an answer for 'water' (I've seen this before). I am not certain of the 'source of fresh' bit.
'ultimately this deep sea' is the wordplay.
'ultimately' indicates one should take the final letters.
The last letters of 'this deep sea' are 'SPA'.
'is a' is the link.
Can you help me to learn more?
(Other definitions for spa that I've seen before include "Curative resort" , "Heated bath" , "E.g. Harrogate" , "Hydromassage facility" , "place of treatment" .)