Not sure how much it matters for the difference in risks after dilution, but beta radiation probably has short reach in water compared to air.
From the perspective of the people releasing the radioactive water, evaporating it would give them larger volumes of a material that is more difficult to manage before it disperses and in medium where more of the radiation reaches further.
Seems like the main environmental concern ought to be absorption into tissues though, and it's not at all clear to me that highly active marine ecosystems with long food chains is the preferable alternative there.
From the perspective of the people releasing the radioactive water, evaporating it would give them larger volumes of a material that is more difficult to manage before it disperses and in medium where more of the radiation reaches further.
Seems like the main environmental concern ought to be absorption into tissues though, and it's not at all clear to me that highly active marine ecosystems with long food chains is the preferable alternative there.