If they're trained lifeguards, Wild Wadi and Aquaventure often hire.
Maybe google and contact AIESEC Dubai. They are an international student organization and have internship/trainee positions for students with companies like PricewaterhouseCoopers.
ETA: I think you may have to be an AIESEC member to apply for positions. They have a selection process with interviews, group activities, et cetera.
<em>edited by em2san on 28/03/2011</em>
Some of the schools will take them on, but why Dubai? SO many great things you can do in a gap year Dubai would be my last choice for someone of this age range. Also some of the hotels take gap students on as Life guards etc.