Water can change things. It might easily combine with the corium to form various hydrates.

Hydrates contain hydrogen, which is a neutron moderator. To the extent that corium's complex decay involves neutrons, changing the neutron energy spectrum (what a moderator does) could change the decay pathways and durations.

Electronic ("normal") and nuclear chemistries can interact.

Edit: Also, water dumped on hot stuff can crack it. Cracks mean that regions of material have changed their geometric relationships, which affect nuclear decay.