Honestly, maybe not that much. Mean ocean temps would not move that far - the oceans are massive - and the water that was vaporized would quickly precipitate back down into liquid water again. Water on Earth is on a three-state equilibrium, which is a hugely stable system. All the agricultural and industrial output of humans over the last ten millennia has only recently started to **** with that equilibrium. The isotopes you put into the water wont change much radioactivity because water is a pretty good radiation shield too. Maybe you kill off aquatic mammals by defining them, and that might upset the ecosystems and maybe that leads to a food crisis? Or an over/under-abundance in some trophic level? But by this link in the chain any guess you can make is too uncertain to be worth anything without investing a lot in serious modelling.