That wasn't very clever! But you don't want to hear all the stupid things I have done over the years.
I still haven't gotten over forgetting to take out the rag I had stuffed down the front of the timing cover on my datsun L20B when I had taken off the timing gear. After reassembling everything and trying to start the car, I wondered why it would barely turnover before a strange pop noise. Look at engine to find the rocker cover had exploded out, but that just led on to other things....... etc etc Perhaps we need to start a thread of stupid things we have done.
Anyway, back on topic. You will have to drill out the remainder of the bolt with a smaller drill bit and use a bolt extractor tool if you don't want to bugger the thread. Chances are you will bugger it so may need to use a tap to clean it up.
Looks like the water pump has been leaking for a while. Just pull it off and see where the leak is coming from. Probably just needs a new gasket or seals, rather than a whole pump. The thermostat housing often leaks and is a real prick sometimes.