We've established a hundred times over on Retrospace that cheesecake sells records. More often than not, the swimsuit/beach scene was the go-to route for either compilation records or easy listening. Those type of albums tended to feature exotic (i.e. non American, often tropical clime locals) music - and what better way to illustrate your LP of Brazilian or Tahitian than with a babe on a beach?
Also, having a dame in a bikini maximizes the skin potential. You can't have her naked, and a gal in her bra and panties might be a tad risque for an album cover. So, how better to show the most epidermal surface area than the bathing suit? Elementary, my dear cheesecake peddler.
Of course, from the right angle you can get away with no bathing suit at all!
I've shrunk some of these images down to conserve space; click to enlarge
This has to be Ali McGraw (with blonde hair, oddly enough).
The last four courtesy of Epilectic