When I'm casting I dress up like my life depends on it. Heavy cotton shirt, heavy cotton pants, leather calf high boots, a firehose canvas apron, gloves, a hat and a face shield. I assume that there will be a spill. I assume that water will get in the pot. I assume the worst.
Set up your work space. No children, no pets. Dress out. If you're melting scrap lead, wheel weights, etc., the first pot will be warming up from room temperature. That one will be fine, the heating process will evaporate off any water. It's lead you add later that might have some moisture on it that will cause a problem. Preheat any lead you plan to add. Get it hot enough you can not handle it without gloves before you add it to your melted pot.
It is known as getting a visit from the tinsel fairy. Moisture, even a small amount in a bucket of wheel weights, is a serious risk when casting. A damp ladle, perhaps carelessly set on a wet towel and then dipped into the pot will do it, too. The liquid lead is 650 degrees (F) or hotter. Added water is transformed instantly to steam. That expanded steam will leave the lead pot with authority.
Lead on the ceiling of the garage or the hood of her car may not be pleasing to the spousal unit. Lead on your arms, face, head, or hands will instantly result in severe burns. Here's one more link from the Cast Boolits forum on the topic.
Here's a video of a guy melting lead pipe sections with a very small explosion at the end. I think he had just a little moisture in that piece he adds to the pot.
Be safe and cast for a long time.