Jellyfish the size of dinner plates in Australia, for example, or swimming outside the shark nets in Hong Kong.
Britain's Kerri-Anne Payne was the first athlete to try out the Serpentine course Wednesday, and was full of enthusiasm.
"It was really nice to get in there," she said of her morning swim. "I had to fight with a couple of ducks though to get round the buoys. But other than that it was pretty good." She also found swimming through the reeds rather interesting. "I kept getting a little tangled in them, but I've overcome that now."
As an open-water swimmer racing in natural settings rather than the controlled environment of a pool, being prepared for the unexpected goes with the territory.
"I can assure you that as an open-water swimmer, I've swum through way worse things than reeds and a few ducks," she said. "It just depends on the person and how strong-willed you are."
Some of Payne's more interesting swims have included the 2007 World Championships in Melbourne — her second ever open water swim, which she shared with "thousands and thousand of jellyfish in the water, all the size of dinner plates. It was very difficult, to be honest."
Another memorable occasion came in Hong Kong, when she found out the big buoys the racers were swimming outside were in fact shark nets.
"I kind of made sure I stayed in the middle of the pack for that one," Payne said.
Then there was the river in China.
"Everybody keeps mentioning the dead horse. There was no dead horse. Just a dead dog."
So reeds and ducks? Really not a problem at all.