大学英语课文的一个句子分析AlexHaleyservedintheCoastGuardduringWorldWarll.Onanespeciallylonelydaytobeatsea--ThanksgivingDay--hebegantogiveseriousthoughttoaholidaythathasbecome,formanyAmericans,adayof

　　大学英语课文的一个句子分析

　　AlexHaleyservedintheCoastGuardduringWorldWarll.Onanespeciallylonelydaytobeatsea--ThanksgivingDay--hebegantogiveseriousthoughttoaholidaythathasbecome,formanyAmericans,adayofovereatingandwatchingendlessgamesoffootball.HaleydecidedtocelebratethetruemeaningofThanksgivingbywritingthreeveryspecialletters.

　　这是全新版大学英语第三层第五单元的一个句子,其中“Onanespeciallylonelydaytobeatsea”为什么要用tobeatsea,用beingatsea可以吗,或者只用atsea可以吗,tobeatsea在句子里做什么成分?