Pros
You get to work outside all summer long with similar aged people and there is room for growth. Pay is good if you work hard. If you impress the right people, the pool board, swim team parents, your supervisors then you'll have a great summer or two. Oh, you also get a great tan.
Cons
Starting out they will probably place you at an apartment pool. Avoid this at all costs! They leave you alone at an apartment pool, which no one comes to, for 8 hours a day with no breaks. Make sure you work at a swim club with multiple guards on duty at once.