For many pool and spa companies, it is impossible to employ full time adults for all positions because there is simply not enough work year round. Part time college and high school students are the answer. You need the summer help and they need the work.
Because teenagers have busy lives, one obstacle to overcome is scheduling. Consider a schedule swap program. Allow them to swap with each other whenever they need a day off for something special. To implement a swap program, you need to establish some simple rules: No overtime is created due to a swap, the person must match your training or job title, the manager must approve it, and the entire staff must be informed. It is simple and everyone is happier and more dedicated to the job because of the flexibility. (By the way, adults like this program, too!)
Students will also appreciate the job more if you give them responsibility. Consider letting them be responsible for the water in the displays, the water lab and supplies, or updating shelf tags when prices are changed. Depending on the number of students, spread the responsibility around to match their talents and train them properly on their tasks so they are comfortable and confident. Nothing demotivates a teenager more than coming to work and only cleaning things in between customers.
Make it a point to teach your students good work ethics and about business in general. Since they are young and just getting started in the work force, they need guidance, education, and respectable role models.