I think i have an infected pimple, it's really red and swollen and angry and has seemingly endless amounts of puss. is swimming in a lake w this safe?

No.: Any time you have an opening in your skin, this gives the bacteria in your environment a chance to enter your body and causes infection or worsen your current infection. The lake water is not bacteria free. I recommend staying out of the lake if you have a pimple full of puss as you described. Wait till it heals. If the pimple is getting worst, a doctor will needs to see it to help you.