I wanted to include a link to a great tutorial video that shows the process of creating your own Sims 3 diving area, but the linked content is now marked as ‘Private.’ All I have to offer is my notes, so take them and create, and please let me know if something here isn’t clear.
Creating a Diving Area (from scratch):
Enter the following cheats in order:
From Edit Town, place any size lot into any darker area of water.
Change the lot type to Diving Area.
Go into Build/Buy mode and and use the ‘Flatten Lot’ terrain tool.
Return to Edit Town and select a save game file name (save before the next step).
Return to Main Menu (after saving) and then return to the dive lot build saved game.
Go into Build/Buy mode and go down a level to get under water, make sure it looks okay.
Return to the water surface, go into Buy mode and click on Debug objects tab.
Misc Objects: (specify Island Paradise to limit the objects). Select a Scuba Buoy and place it near the center of the lot.
You can also place 1 or more Surface shark spawners at this time (optional) near the Scuba Buoy (buydebug: fish spawners: surface shark). They can also be added later.
Looking directly above the Scuba Buoy, drop down to the under water level and go back into Build mode.
Using Terrain Tools, lower the terrain under the buoy a little bit and then lower further with the Level Terrain tool.
Return to the surface and move the Scuba Buoy a little bit. It should now be sitting properly in the water.
Go back under the water and use ‘Flatten Lot’ to make the ground level even.
IMPORTANT: Once you have the Scuba Buoy properly set, DO NOT TOUCH IT again. I’ve read that if we move it after it’s properly set, it could be messed up. Then the terrain would have to be adjusted again, and it’s best we not have to do that.
Sculpt the lot any way you like using the Terrain Tools.
Use different terrain paints to improve the appearance of the under water lot.
From buydebug, add Under Water objects to the lot. You can also use objects in Build Mode, Misc Community Objects.
Treasure Chests: buydebug, Misc, use Island Paradise filter and select underwater treasure chest(s).
Place chest(s). Shift/Control/click on chest to bring up the special menu.
Set Required Diving Skill Level
Caves: To customize Shift/Control/click on cave to bring up the special menu.
Set Name of This Underwater Cave, etc.
Spawners: find in buydebug and place around lot.
Underwater shark spawner
Other underwater/sea life spawners
Under rock/gem are bottle spawners
You can check under all buydebug tabs for objects to use on diving lot.
Kelp and underwater plants
To add visual effects underwater: from buydebug, Misc Objects, select Fog Emitter (blue/green color). Place & add codes.
Set Visual Effect…Codes:
When you are finished under the water, return to the surface and set the required skill level for diving.
Control/Shift/click on the Diving Buoy and select a skill level that is 1 or higher (don’t select the 0, no Sims will be able to dive).
I found a tutorial written by Weealbet, the person who created the video that is no longer available. There are screenshots included, which is a big help.