This is my “Underwaterapplehouse” I am working on. I just have to add some plants to the ground, but it allways needs 13min to render (Blender Internal) my PC is very old 
To boost rendering time, get more RAM. Anyway, I suppose your got the idea from Spongebob Squarepants?
Well I love the idea!
If you can’t buy a newer pc, (as is the case with most of us here, heh), try searching for the render region tool.
I don’t remember if it’s already in blender or if you have to download it as a script from the internet. I hope it works.
No I don’t tought about Spongebob, but I know what you mean. I will buy a new PC after christmas so I wont have this problem anymore 