Block it off by stuffing a soap-less
stainless steel scouring pad or two around the chain. You get about six of 'em for three bucks. That lets the chain locker breathe but keeps out roaches and mice.
For stopping seawater when on a
passage, I put a
plug of putty in the chain link at the mouth of the hawse pipe, then cover the mouth of the pipe and the putty-filled chain link with Denso tape. No
water gets past that.
Wipe it all down after the
passage with a rag dipped in kerosene.