Not only are some girls afaid to jump in the water, disproving the claim that this is a "girly thing" that all girls do, but a good number of boys aren't afraid of this either, so neither part of this one is correct.
This isn't a girly thing at all. Idk why people call this a "girly thing". It drives me insane. A lot of these "girly things" can be guy things too. So don't go around call it a 'girly thing' because it's not.