I would guess that it depends on the lake…
The main sources of risk during and after floods come from combined sewer overflows (CSO’s) where sewers which carry both rainwater and foul sewage become overwhelmed by rain and flood out of manholes, emergency overflow outlets etc – that can then run anywhere, but is quite common in forming small pools in urban areas. I suppose that could run into a nearby lake, but the actual volume of water isnt likely to be that high compared to a lake (rather than a pond), so the dilution factor will probably reduce the risk, unless you are right by the source.
The second possible source would be polluted river water entering a lake, either directly as the river flows in, or through floodwater reaching the lake. This could be more problematic, as during heavy rain sewage treatment works can discharge untreated sewage into rivers, because they cannot deal with amount of sewage flow coming in (see above CSO point) and the dilution of fast flowing rivers is considered sufficient to avoid the worst environmental damage. Sewage treatment works can discharge pretty large volumes of this untreated stuff into rivers, which happens during and just after rain, so while it might not kill all the fish, i wouldnt want to swim in it especially near a big sewage outfall.
As i remember it, this is what made David Walliams ill. Leptospirosis, if i recall correctly, is mainly a confined spaces issue for people who go into sewers and pipes. Rat urine is pretty concentrated in that context, so less risk in lakes really, but i’m no doctor so maybe don’t take my word on that….
So, some lakes will be fine, but some may have a risk depending on what flows into them from rivers or from surrounding sewer networks. Maybe avoid swimming near big pipes spewing brown water? But then, the pool is probably a bit warmer anyway.
