In theory yes. In the real world I would not be surprised to hear that there are higher concentrations in specific areas due to currents and whatnot (ie it would take a while to evenly spread out)
East china sea: ~770,000 square km surface area; 350m average depth = 269,500,000,000 cubic meters of water
First discharge = 7,800 cubic meters over 17 days == approx. 459m^3 released / day == 1.70315399 x 10^-9
They might find five or six extra tritium atoms per liter of water.
Does this one off make it okay to dump into the ocean generally? No obviously not, the ocean is not just a magic dumpster to make bad things disappear. But in this case, tritium already exists naturally in sea water, and the introduced concentration has already been minimized.
If your personal heuristics for avoiding risk push you towards avoiding Japanese seafood over this, that's fine, but you should actually be avoiding all seafood.
My intuition could be way off, but I think there's more than 10x more water (even at some ridiculously thin surface depth) between Korea and Japan than there is in the Fukushima tanks.
Wikipedia says 210 km³ flows into the Sea of Japan yearly. The article says the total stored water is "one million metric tons", which is... probably within the measurement error of the total SoJ flow.