"Watch out! There is a shark!"
Translation:조심하십시오! 상어가 있습니다!
This exercise is about the deferential speech level formal, polite.
One of the uses of this level is to communicate with the general public (as seen in this case).
⊙ For the imperative form (command), use: VS + -(으)십시오
Watch out! = 조심하-십시오!
⊙ For the declarative form (statement), use: VS + -습니다 / -ㅂ니다
There is a shark = 상어가 있-습니다.
Note: Your translation is the familiar speech level i.e. used on a more personal basis. (Polite but casual).
That said for a command a ~해요! ending may sound coarse. It would be better to use ~하세요!