I found a way to export diving areas from Isla Paradiso to other maps!
Here are the steps:
There are 3 cheats to type using ctrl+shift+c
First cheat: TestingCheatsEnabled true
Second Cheat: BuyDebug On
Third Cheat: EnableLotLocking true
After these 3 cheats , you should be able to edit/move diving areas , also set a lot as diving areas. Then export a diving area, and open other map.
Put a 64x64 lot in the sea and choose lot type as diving area then put the exported one in it. It should show some mess like this picture. You don't need to level the terrain. But remember to check again lot type after you put diving area.
Just ignore it , save and quit to main menu and reload the map. Then you should have a functional and normal diving area now, enjoy it! It's a diving area clearly in Sunset Valley.
By increasing your skills you can watch sharks under water
Diving area in Lucky Palms
Diving area in Sunlit Tides
I found the problem in some worlds , like Barnacle Bay. And if you edit the lot you'll know the reason is the underwater ground level is too shallow. So you need to choose deeper place to put diving area (I think that's why Devs don't want us put diving areas in other maps). Or follow the steps of this tutorial.
My method is easier way, and the list of succeeded map:
The maps that may cause problem :
The maps that won't work:
I'll update the list after I tried different worlds, and I hope simmers can reply your success/failure in other maps.