Anyone who thinks Jimmy Buffett would rather be sailing this holiday weekend knows him by reputation only. Last night at Merriwether Post Pavilion (where he returns tonight), Buffett was in his true element, appeasing the rock 'n' roll hunger of thousands of fans who were vocally primed for his celebration of sun, surf and suds.

Buffett hasn't had a hit in five years but it matters little to his fans who greeted even a beer commercial he wrote recently with a standing ovation. What matters, really, are the old songs and the easy, charmed life style they suggest. "Margueritaville," "Changes in Latitudes . . .," "Cheeseburger in Paradise," all delivered with power and enormous enthusiasm last night, hardly have lyrics for the ages. But in Buffett's hands, with a fine "Coral Reefer Band" behind him, these songs are quickly transformed into rousing summertime anthems.

The evening's only drawback came during Buffett's all too brief acoustic set. Though it included the lovely "Banana Republic," it focused on an especially long-winded version of "God's Own Drunk." Buffett could have easily spent the time on some of his better ballads, but then subtlety wasn't much in demand last night.