When I said half life, I made a mental shortcut that it degrades into harmless compounds.
The 12 days just means how long the body keeps most of tritium.
You are talking how much radiation the water causes and that it is smaller than radiation from banana, and I'm talking that this "banana" stays in your body for 12 days and part of it your body integrates by replacing your hydrogen with its radioactive counterpart.
You work with radiation, but this isn't just about radiation, but also involves organic chemistry and metabolism.