That's what my dad used to joke about my mom when they first married, she was so bad at cooking she could burn water. Well, probably not that bad, but she learned how to cook from My Grandma Florence, while my dad learned from my Grandma Mabel. They had totally different cooking styles. Grandma Florence would measure EVERY thing out to the last drop, while Grandma Mabel couldn't duplicate a recipe if her life depended on it. She usually added a bit of this and a bit of that. I really wish I'd watched and written down some of her recipes as she did them. She made some darned fine cookies.
But back to "burning water". I think the joke is about new or fairly inexperienced cooks who tend to get distracted while cooking. I know I've done my share of that (story about the cast iron dutch oven I thought I killed some other time). The key to cooking is to have patience and learn to pay attention.
I started cooking when I was 5 or so, tho for the most part it was under strict supervision from my parents. There was the time when I wad 6 I decided to surprise them with fried eggs. My dad was great, said they were the best blackened eggs he'd ever had and this was LONG before the cajun food craze hehehehehehehe. Then he proceeded to show me how to properly fry eggs.
Boiling water, it is the simplest thing in the world, yet can sometimes be a frustrating thing. The key is patience, as the saying goes, a watched pot never boils. Took me the longest time to get that thru my head. Yet you have to be careful because you can boil off the water and ruin a good pot. So when it comes time to cook, try to avoid having other stuff going on. Last thing you want to do is find out how well your smoke detector works LOL.
The trick to boiling water is to fill the pan 3/4 full, turn the heat on high and then back away. Use the time to prep the rest of what you are cooking, just keeping an eye on things. When you see steam coming from the pot and if you look in and the water is rapidly boiling (that's why it is called "boiling" hehehehehe), it is ready to add what ever it is you are adding. Once added, you can keep the heat on high, but be careful, you can have a boil over and that makes a mess. Until I was comfrotable with the concept, I usually turned the heat down a notch, to keep a slight boil. Now I tend to keep it at high, mostly becasue when you add your pasta and such, the temp drops some and it does take a bit longer for things heat back up.