A lot of fish where the water is shallow (5)
I believe the answer is:
shoal
'is shallow' is the definition.
(similar in meaning)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for shoal that I've seen before include "Collection of fish" , "Fish grouped" , "A great number of fish swimming together" , "School of fish" , "Mob" .)