My swing keel weighs 650 lbs.similar problem. The little animals weld it up tight in as little as 24 hours.
First loosen off the winch slightly and go sailing down wind. Bouncing the boat off the wakes and waves is usually enough to free it up. You will hear a slight thunk and the cable will be tightend up.
Didn't work loose? Now you need to swim.
Get a piece of flat chain. Bolt it together to form a loop. Loop it over the back of the keel and use a 5 or 6 foot long pry bar through the loop to pull the keel down. Protect the hull as necessary.
Still didn't come loose? Get a piece of re bar or solid steel round stock, remove the winch, go to shallow water so the keel can't fall off the boat and drive the keel down using the bar stock as a drift punch. Big hammer! This method will damage the keel some and it will need to be fixed sometime.
Don't leave the boat parked with the keel all the way up. If you leave the keel down a few inches just the motion of the boat will discourage the beasties. Yes the thunk-a thunk is annoying. Overnight you should be able to pull it tight but don't forget it.
Yes I have used all three methods. Took me a while to figure out what was happening!