I was recently crossing a lake that is commonly used for swimming and recreation. When I was swimming back to the shore, I was constantly nervous that my bag would be stolen, along with the keys in it. The lake spans several hundred metres, so there would have been opportunities for thieves taking our stuff. Luckily, everything was still in place.
I want to find a way to take the key to my flat and bike with me while swimming without any chance of losing them. I do not need more than the two keys and a towel (the mobile phone will stay at home).
Edit: Thank you all for the overwhelming amount of replies! I try also to follow all the comments and still don't know which of the replies I should pick for marking as the answer because so many of them are so great. :-)