That doesn't really work because marine life is good at filtering and concentrating a subset of the elements that are in spent nuclear fuel. There are already ocean fish that are too poisonous too safely eat because of (coal-emitted) mercury pollution—and that's only 100,000 tons of mercury, total, in the history of human industry [0]. If you dig in to the hard numbers surrounding spent fuel, it's a much, much more toxic and difficult problem than mercury—diluting it in the oceans is a complete non-starter.

[0] https://en.wikipedia.org/wiki/Marine_mercury_pollution