Silverfish don't eat blocks from what i've heard, they pop out when you destroy one. Also, Endermen are extremely rare and only teleport short distances, so building a house just about 10-15 blocks from the shore would be safe enough. Besides, you have to look at them first.
I don't think I'll ever be able to go to the bottom, the random clicky noise it made is already going to make it difficult to sleep :'( From the sudden flash of darkness, it looked like the shadowlurker.
You could include a dungeon part which is like a series of dungeons which have skeleton and zombie spawners in the walls, amke the floor out of cooble, though, or creepers and spiders will spawn, too, removing the "abandoned prison" feeling.
Did it use the same name? because sometimes after you delete a world and use the same name, the other world might still be in the files, so it would be re-used.
I think what Notch should do is keep the survival mode, and also have Adventure mode, so when you load up minecraft there would be: Survival, Adventure, and Multiplayer. along with the other things on the menu, of course ^.^
