Tem um tubarão?
"Don't get in the water" or "don't get into the water" should also be accepted
My feelings exactly
em + a = na.
Is that right?
Yes... in english we dont add any preposition after "enter", but in portuguese we say "entrar em".. and em+a = na
Is the sentence "Do not enter in the water!" Incorrect? Why?
I could enter into the water (however redundantly), or go in the water, but it doesn't feel right to enter in the water.
Enter already means "go in". "Enter in" is redundant
Is this subjunctive?
Imperative negative, which has similar conjugations to subjunctive.
Why not "não entrem"? (Vocês)
It should be accepted also.
What is wrong with nao entra na aqua please?
You should say "Não entres" (tu) or "Não entre" (você), but not "Não entra".