Stress
Working at a country club pool probably isn't a very stressful summer job. Yes, you need to be vigilant and constantly on the lookout for people in need, but most of your day will be fairly easy.
Working at a crowded public pool, summer camp, or water park? There will likely be more excitement, but there's also the stress of being responsible for kids and weak swimmers. Hopefully everything works out, because having to save someone and administer CPR is extremely stressful, especially since there's no guarantee it's going to work.
The beach is the most stressful location for a lifeguard to work. Think about it—you're responsible for itty-bitty people being engulfed by ginormous waves and sucked into the sea forever. Yeah, that would stress us out, too.