The ocean's average radioactivity is 12Bq/L, and there's a _lot_ of water. A single banana is 20Bq already.
Dump a few million litres of 10000Bq/L water in the ocean over a few years, and the average radioactivity goes to... Still 12Bq/L
This is unlike, say, naturally radioactive carbon or potassium isotopes, that have a relatively constant concentration in the animal's body over it's life time - if a fish eats some high potassium food, it will excrete an equivalent of his own equally radioactive amount to maintain homeostasis.
It's for this exact reason the net "banana dose" of radiation, unless you are potassium deficient, is in fact zero.
https://twitter.com/TristanKamin/status/1694248709180084514
TEPCO did not detect any iron 55 or selenium 79, yet they are included in their estimates, at the worst possible doses, explcitly for safety. The amounts rejected and the radiation it exposes you to is about as harmful as living a few floors higher up that what you do. It's nothing.