Dont forget that this is just like any other interview. Dress professionally, act mature (even if your not :) and dont be afarid to ask them questions. Most of the time though they will ask you things like your past expirence with lifeguarding. How many saves you have had to do, if any explian them. They may ask you to run through what you would do in different situations. They may ask what you can bring to the job and how you would be a asset. But also ask them questions like how many rescues have been done in the past month? year? Ask if they have a AED, ask how often they have inservices or drills, ask about thier procedures and dont be afaird to voice your concerns about somthing you dont like.