Manji doesn't remember hearing that because Hickey hasn't actually told Manji he can't swim. Saying something like that involves admitting weakness. And you only admit weakness when it might be advantageous. And not getting pushed in the lake? Absolutely advantageous.

That doesn't mean he likes it, though. Hickey blushes, flushing a bit, scowl on his face as he grumbles, "Yeah. I can't swim. Never had the means or opportunity to learn back home. And anyway, even if I had the means, you don't actually swim in the Thames."

London circa 1840 is very gross.