Body of water in addition almost filling region (4,3)

'body of water' is the definition.
Both the answer and definition are singular nouns.
Perhaps they are linked in a way I don't understand?

'in addition almost filling region' is the wordplay.
'in addition' becomes 'also' ('also' can be a synonym of 'in addition').
'almost' means to remove the last letter (most of the word but not all of it).
'filling' means one lot of letters goes inside another.
'region' becomes 'area' ('area' can be a synonym of 'region'**).
'also' with its last letter taken away is 'als'.
'als' placed into 'area' is 'ARAL SEA'.

Can you help me to learn more?