Minecraft wiki entry for Shelters, associated text: Make a pit of any shape or depth, but something that's 2 blocks deep is recommended, so that you don't sink to the bottom and drown. You could also place half slabs on the bottom of a two block deep pool so that you get the feeling that you're fully immersed in the water but without fear of drowning. Fill it with water. And now you've got your very own pool. And if you're extra careful, you can make it with lava instead. Also it lights up that area of your base and looks good. You can add glowstone underwater as 'lights', or lava with glass over it to make it into a hot tub.