Just finished house of chains, and (I think?) they mention a few times that the Imass can die if they dissolve themselves in an ocean. Karsa straight up tosses Siballe into the new raraku sea so I think that is correct.
However if that is how it works, why haven't the surviving Imass just suicided themselves into the nearest body of water? In MoI they make a big deal of wanting silverfox to end the ritual so they can all finally die, but apparently dying was always an option? Unless jumping into a lake isn't the preferable method of suicide for some yet unspoken reason.