Getting someone's attention to share

Hi! New guy to the forums here, and pretty new to lap swimming too -- I've been doing it since May sometime. More than once when I've wanted to join someone in a lane I've found it difficult to get their attention. At first I thought people just didn't want to share, but then I paid attention while I was swimming and realized that when you're looking at the bottom of the pool it is indeed easy to miss someone standing on the edge. So what do you do? I've taken to dangling my feet deep enough that they're hard to miss, but is this obnoxious? Does anybody have pointers for getting someone's attention without annoying them when you need to share a lane?