Notice that Seymour is very pale and doesn't want to get any sun on the beach. Muriel, on the other hand, is inside because she's sun-burned so badly. Getting too much sun is sort of equivalent to getting burned by material pursuits. Or, it could be equivalent to being jaded by experiences in the world. If this is the case, Seymour has maintained his spiritual purity or his youthful innocence, while Muriel has not. You might also want to consider the woman in the elevator with zinc salve on her nose, or the fact that Sybil is being slathered with sun-tan oil when we first meet her.