Paul, the only time I've been rammed has been well above the water line. Unless you are sheared into by a plumb bow, or run into debris, that's most liekly how you would be hit. So the first move is to heel the boat the other way, try to get the hole above waterline so you can inspect it. After that, you rig a crash mat (collison blanket) as best you can, if you need it.
If you have old sails, doubling them over and having a sailmaker put a heavy cringle in each corner is one way to make them. Better if you take a cockpit cushion (closed cell foam covered in vinyl) or settee cushion (same kind) and have the cringles put in, or one of the commercial mat-like fenders that you can buy with cringles already in them. A couple of extra cringles gives you more option in how to apply it. Remember the water will be trying to pull the whole thing off and apart if you are making way...so more and beefier cringles are probably a good idea.
A couple of cans of urethane foam probably could also be used as a sealant, i.e. applied (from inside, to the outside) around the hole. It seals up and bonds nicely, even in water. Mess to get it off, but I'd rather seal the hole first.