As indicated already in our Forum and in our community, Miniclip strongly suggests upgrading any Guest account to avoid any data loss. Why?
You make all of you friends jealous with your perfect dives in your Guest Account and, suddenly, you lose everything due to some freak accident. There are several ways to lose one's account, like signing out from your account, losing/changing your phone, cleaning your cache, etc. Since the game is saved only in your phone and nowhere else, then at least with all those situations you will lose ALL the information (the level you are at, purchases, coins, items, etc) related to that account.
And in most of the cases, there is no way for us to recover your Guest Account, so it's lost forever.
How to prevent this?
Go to your settings and, far down the screen, you'll see the "Connect" icon. This will connect your game to your Facebook account, saving the progress you made so far. This will also automatically save your game.
Now you can dive freely and safely!