Insimenator.org

I downloaded some nice beach houses.  I installed one and bought it.  Put it in one of my downtowns.  But the part that was supposed to be ocean was just an empty hole in the ground.

Do you need to put these houses by the ocean?  The three base cities don't have any places where I could do that.  Do I have to create a new city?  

If so, can I move my sims from the other cities to the new one?

You could probably just add water by going in to build mode and the terrain tool and using the water tool.  Don't quote me on this but it's worth a shot.

Why would you put a beach house in downtown? Seriously, if you see a lot with a lot of ocean and the lot is partially underwater, it was designed for Bluewater or the other hood with a high water table.

Beach houses are built for the Bluewater District, only if the land is sitting higher than regular level and the land is sloped deep so neighborhood water can run in the lot.  Do you have "Open for Business"?  That is where the beaches are located for the desired effect. Hope this helps. There is some beautiful custom made neighborhoods that are offered all around the sims2 community.  To answer the second question, you can move your sims to added on neighborhoods.  To explain, say for instance, you play Pleasantview and you have the Downtown, and Shopping Districts, then you can add a new neighborhood to your Downtown or Shopping District and you will keep connections your simmies have in Pleasantview. Don't move the whole lot though, the simmies will lose all connection that way.  I hope it helps with your questions.

I second what lewis said. I don`t have OFB but I do have a custom neighborhood terrain that allows me to put houses on the shoreline. It`s from MTS2, some kind of island.