SOLVED. YA Fantasy series. Uncle gets his nephew to go help him save other dimensions. First book is set on a underwater/floating civilization dimension. [s]

[Daniel]

The main character's uncle is part of a secret order whose mission is to save other dimensions. They go through natural portals in caves to go to the other dimensions. In the first book, they go to a dimension that is entirely submerged underwater. The civilization is on floating cities and the uncle dies at the end of the book. When the nephew tries to go back home, his house is gone and so is any trace of his life. The only one who seems to remember him is his girlfriend, who helps him in later books. In later books, he goes to a dimension run by feline people whose main energy source is crystals, a dimension where almost everyone is plugged into a Matrix sort of system voluntarily to escape from reality, and he needs to get people to start working on bettering their world instead of being sucked into a false one and he fails in that dimension. There is also a book where he goes to the next century, because that counts as a different dimension apparently, also one where he goes to the last century to make sure the Hindenburg crashed. In each book, he doesn't know what exactly he is there to fix.

[Becca]

The Pendragon series by D.J. MacHale? The first book is The Merchant of Death but it looks like it's the second one, The Lost City of Faar, that's on the flooded world. The Matrix-like one might be The Reality Bug and the cat people one Black Water.

[Daniel]

Yes thats it! Thank you! I kind of got the books mixed up so thanks!