Frozen water pump bolt

I've had the timing cover oxidize to the bolt before too. Most likely as mentioned you'll have to cut the head of the bolt off then get the cover broke loose and work it off. The bolt should thread out then. Now I use thread sealer on the threads and anti seize on the bore of the timing cover. I use as little silicon as possible, seems it shows up some where I don't want it to later if I use to much.