I do not see why you would limit your concern to sea water.
Sea water evaporates and becomes rain. The hydrosphere mixes pretty well, with the exception of ground water, which can take a long time to cycle.
But tritium is just an isotope of hydrogen, so, as part of water, it goes through the rain/snow cycle. The tritium in sea water will soon be fairly evenly distributed in all the rain and river water in the world. Fresh water lakes will take a little longer, and it will take much longer to distribute in ground water.
With a half life of 12.32 years, in 50 years it will be only at 1/16 the level it is at today.
That’s why I mentioned that making drinking water out of seawater in CA is out of the question, but this is exactly what CA wanted to do at one time. CA must forget about the project altogether.