I'm only here for the summer.
Lifeguards are traditionally seasonable employees that are only hired during the summer; these jobs usually require less dedication/training. Office jobs, on the other hand, are usually not temporary. It's funny to imagine this employee's surprise to find that his boss a seasonal employee (because he's dressed as a lifeguard); this might mean that the employee's work will have less oversight or that the company rotates management so frequently, they might as well be seasonal.