Question

Why can't we swim in Saturn's moon Titan's lakes?

Answer 1

what are the lakes made of?

Answer 2

A quick google shows that the lakes are "bodies of liquid ethane and methane." Those don't sound hospitable.

Answer 3

This is an open question, do you need more information regarding this?

Answer 4

Pressure, temperature, substance
Pressure: The liquid on Titan has varying pressures that could be as lethal as a geyser
Temperature: The liquid can be really cold, which can cause fast hypothermia
Substance: as mentioned by Shadow, the liquid on Titan is made of both Ethane and Methane, substances that won't feel particularly good on your skin