Hi there, all I did was translate the tutorial all credit goes to the original people who posted this tutorial which can be found here
Before you even enter your chosen world you must enter these three cheats.
In Edit town place a 64 by 64 lot as deep as you can into the ocean
Change the lot type to community lot
Change the type of lot to diving area
Click on build/buy mode
In build mode use the flatten lot tool
Go down to the ground level and place some seaweed, you will find it in buydebug mode it will be under underwater objects. Everything will appear to be a matt blue colour when placed, but don't worry it will soon all be normal again.
Return to edit town and demolish the lot only once
Now reselect the lot as a diving area
return back into build/buy mode
Now you can decorate your lot with beautiful buydebug items from the underwater section.
Don't forget to place the buoy to select the capability of diving skill the sim must have to dive there. To set the danger level simply hold control and shift in and click on the buoy to select the levels.
Go back into edit town and add a name to your lot, once done click on the lock to lock the lot again.
Now your lot is ready
That concludes their tutorial, they also linked to a video on youtube showing their lot in action here
I hope this helps those of you who can't translate the page, remember I only did translating I take no credit
for this tutorial