The person renting can be responsible for the full cost of the #Divvy bike *plus fees*, if not returned. Those in the lake were probably stolen at one time.
Renters can be responsible for as much as $1,200 for a lost or stolen bike. Don't leave them unlocked and out of your sight, even for a minute.
If yours does get stolen, merely filling out a police report might not be enough to get you out of that $1,200 fee. You might need to insist, in order to get the case escalated beyond a mere report.