After being a stompy monster for several minutes because I couldn't make my own houseboat from scratch, I did stumble on a way to do it.
I'll feel kind of dumb if I post the information but everyone else already knew... :oops: but, you see, I have Tutorials turned off with extreme prejudice so I don't know if it would normally have taught me to do it.
But in case it helps anybody...
-Go to EditTown mode.
-Select a lot that is partly on seawater.
-Change the lot type to Community/Port.
-Go in to Build/Buy on the lot.
-Click on the water and drag a Pier Foundation in towards shore (it will automatically be the correct height above water).
-Place steps going up onto the Pier (hold Alt as you place the stairs so that it will only use as many/few steps as absolutely necessary).
-testingcheatsenabled true
-buydebug on (Misc category, sort by EP10)
-moveobjects on
-Place a gangway onto the edge of your pier. All four green squares should be on the Pier, and the big blue square of floating wateriness should clear all objects (except, er, seawater).
*At this point you could Shift+Click onto the gangway and create a boat... but that's a debug feature and the resulting boat didn't seem complete at all. So... don't do that.
-Exit to EditTown mode.
-Select the same Port lot.
-Next to "Change lot type" button, there is now a "Place houseboat" button.
Now from EditTown mode the houseboat will be its own lot which you can edit, and the port will be its own separate lot. Taadaa!
DIVING LOTS
EDIT: Oooh! I found the dive lots building tutorial.
http://forum.thesims3.com/jforum/posts/list/689285.page