What's "mess with" mean and when's it used?

Why is "ing" put after "to" here?

This sentence is in present progressive tense and so the verb is put in -ing form to indicate that the action is going to continue on. The progressive tense is not generally used after the preposition "to" in a sentence except after the phrase "close to". Ex: I was so close to falling asleep when the dog started barking. Ex: You are so close to winning the race - keep going!

What is "sec"?

"Sec" is short for "second". Ex: Wait a sec, I have to do something.

Is "when" necessary here?

While technically "when" is not necessary here, the speaker has used it to signify a hypothetical situation or subjunctive mood will follow. Thus, it is better to use it in this situation as it is a good clue for understanding and predicting the rest of the sentence. Ex: What if when you go to work, you get into an argument with a coworker. How should you react? Ex: When you check your email, you should answer emails in order of priority.

What is ‘em?

'em is shorten form of "them".

What's "water" mean in here

The theme of "water" at the beginning of the song is a metaphor. The "river" mentioned in the beginning is the life she has been living, and here, "waters" refers to this metaphorical river. She's saying that there's still hope within her life- within this "river" of her life. "Waters" can often be used in a similar metaphorical way when referring to a situation in life or the way things are. Ex: These waters are deep, be careful what information you look for. => The situation is complicated Ex: There's no hope in these waters. = There's no hope in this situation.