If the lifeguard at the local pool has a Serbian or Russian accent this summer, credit the strong economy, the weak dollar and the cutthroat college-admissions process. If the pool has trouble finding a lifeguard next summer, blame Congress. Where would summer be without foreigners to do the wide range of jobs that American students and other seasonal workers once filled -- waiting tables in the Hamptons, selling souvenirs in Yosemite, mowing lawns and guarding swimming pools?