Casemate guns were backups whose usefulness depended on the sea

Why were wet casemate guns kept when they could be poor firing platforms?

They remained useful in calmer seas and as a last defensive layer against torpedo craft, even though water and corrosion limited them.

The answer accepts that low casemates could suffer from water ingress and corrosion, especially in rough seas. Their value depended on placement, sea state, and the mission assigned to the guns. Many were not expected to duel an equal capital ship; they served as a backup against destroyers and torpedo boats that had passed the fleet’s own screening forces. A battery that was unreliable in heavy weather could still be useful in calmer conditions and in the specific close-range emergency for which it was retained.