avoid dirty lakes?
Is it dirty because of chemicals, or eutrophication? That is the big question. If there are chemicals then you should absolutely not swim in it locks or no locks. That's just dangerous.
If it's just dirty with pond scum, then the extra nitrogen should not hurt your locks. You'll just want to be extra attentive when you wash them afterwards. Pond scum is a bacteria that grows into those huge algal blooms that tends to kill everything in the lake because sunlight no longer makes it's way under the water.
If I were you, I'd look into the surrounding area. Find out what watershed you are living in. Then look into whether or not there are factories along that main river/body of water, or if there are any upstream on any tributaries to that body. Most ponds are fed by underground springs. They still count as part of the watershed.
If fish are dead in there and there is no algae, stay out of it. I'd even recommend calling a wildlife resource and having someone come out and test the water. If the spring is contaminated, it could also be leaking chemicals into your drinking water.
If it is just pond scum, wash VERY VERY well afterwards. I'd even say to get yourself some lavender soap, or other anti bacterial soaps, to kill everything just in case. That kind of bacteria does not live in dry conditions, so once your hair dries, it'll be fine, but for the period of time that it is wet, you don't want bacterial colonies living in your hair. Not to mention that some of those bacteria do not die when conditions change, they just go dormant and then grow again when it's better suited for them.
I guess what I'm saying, in the end, is that you could swim in it, and be extra careful. Or be on the even safer side and find a new pond
Sorry about the rambling