Sounds like head gasket, If the radiator is buggered you might have overheated it.
Blown gasket is characterised by clouds of steam, temp gauge all over the place, oil in the expansion tank, water in the oil, the header tank overflowing and coolant disappearing. (not necessarily all those things at once). A new cat wont cause lots of steam.
On the other hand, if you arent driving the car and only letting it tick over in cold and wet weather then the exhaust will not heat up enough to evaporate the water inside it.
For every pint of petrol burned, an engine will combust(?) a gallon of water.
A cooling system pressure test is the obvious solution but if your Rad is leaking anyway the test will probably blow it completely.