"The fish swim deep beneath the water"
Fish do not swim "UNDER the water", they swim IN the water, whether the water happens to be deep or shallow.
Ha-dagim sokhim amoq mitakhat la-mayim.
Under the water? On the ground?
Why is deep below the water not accepted
It’s probably just not in Duolingo‘s list of accepted answers, because it sounds fine to me.
That is, it’s something a person might say, but Shimkelevine is correct in saying that fish do not swim UNDER the water!
What is there 'lamed' in מתחת למים?
It goes together with the previous 'mem'. For example: הילד יושב מתחת לשולחן. At least that's what I think.
I agree : "beneath" is better; better still: "deep underwater".
Why not עמוקים?
Just because the speaker is using it generically here. עמוק is neither corresponding to דגים nor to מים in the sentence.
I think that "deep" is an adverb in this sentence and not an adjective