Looking for two books

A couple of days ago, I was reminiscing on my childhood and I remembered two books I really liked and want to re-read. To cut to the chase, I remember one book's synopsis vividly and the other, not so vividly but still enough.

They're both Sci-Fi books, one set in present time, the other set in the future or in a dystopia.

Also, spoiler alert.

Spoiler for Book Spoilers to people who are reading the book:

The first book, I can remember the first line and last line of the book going something like "**** and ****'s destroyed the world" and the ending line is the opposite, being them saving the world. The basic summary is that there these two friends/childhood friends. They both humor the idea of time travel. Further in the book, they go to this place where they basically look at a star dying. There, the girl (there's a boy and a girl) picks up a signal from the dying star and it turns out that it's a message from the future.

They decode the message and it basically tells them the end of the world. It tells them the lottery, what they should do and basically retells the future to them. During the first half of the book, they begin experimenting with the message, doing the lottery and buying themselves a submarine. The next few chapters are a blur to me but I remember when the end comes, it comes in the form of see-through jellyfish that destroy your very being on the molecular level when it touches you.

The girl gets her leg disintegrated before they cut it off. The next half is also a blur to me because I, at the time, didn't like the shift in protagonists. At the end, I strictly remember them (the first two protagonists) living the rest of their lives in their submarine, sending the message through the dying star in the past and then they got an idea "change the message, change the past" is what I remember them saying. Then the first line is altered "The beginning of how **** and **** saved the world started like this".

That's basically what happens. I did a huge abridge of this though and I didn't do the book any justice. Then again, this is 10 years ago.

The next book is set in the future and underwater. And that is quite literally most of what I've got.

Spoiler for Book Spoilers! Part 2!:

I remember there being a boy living deep in the ocean. The city he's in is currently in something similar to martial law, and I think he states that "it has been martial law for several years now". For some odd reason, he has the ability to speak with dolphins. I remember there being a girl showing up half way through the book. The guy is shocked that there are still people living above on the surface. The girl is looking for her brother. In the end, stereotypically, the brother turns out to be the big bad, the terrorist. He also has some sort of power, I don't remember, this was a long time ago.

That's basically it.

Thanks for helping, I'll go do something while I wait.

Looking for two books