1. Make sure you send the letter.

2. Make sure to send the letter.

(Are both the same?)

3. Be sure to drink lots of water.
4. Be sure that you drink a lot of water.
5. Be sure that you should drink a lot of water.

(Are the three sentences the same?0

1 and 2 mean the same, yes.

3 and 4 mean the same thing.

5 is not used; you don't need "should" in this subjunctive clause.