After you get the dive fish, you can go to the cat with his own dive fish sitting on a shell (bottom right corner of the island) and open an underwater valve to enter the temple.
After couple pinball locations and backstory written on the 3 half circled "ancient tablets" The achievement will unlock right before the 4th tablet when you will land on the save flower.
I had a little issue with this one and I also saw couple people having troubles with achievements not unlocking in this game. Had to do 2nd playthrough and the only change was to make sure my account is the only one logged in. Some games are acting strange when you have more than one account logged in and for this game it was a solution I found that worked.