I always hear this phrase "focus 1/3rd into the shot" to make everything sharp and I don't understand how to achieve this. I usually focus on the "main" object, such as a church in the distance and choose F16 or so. I've read though that I should focus 1/3rd of the wya into the shot. I THINK I understand, but what if say 1/3rd into the shot is just water? Do I still focus on the water and everything else would be sharp?

Sorry for the noob question but I'm trying to improve my landscapes etc and this seems to come up often