The crashing waves sounds (there's also creaking wood and the sound of a bullwhip) are at the beginning of the title track, Ship Ahoy, obviously to add some atmosphere to the lyrics of the song, which are about the African slave trade. The recording of these sound effects is stereo, so there was a limited number of things they engineers could do with it. As I've mentioned elsewhere a few times, I spoke to Arthur Stoppe, who assisted Don Murray with the quad mix of this album (and who quickly became Sigma Sound's quad guru) and he explained to me the theory behind the "diagonal pans." Ship Ahoy (the album) has several, including the bass guitar on some tracks, and the baritone voice on some tracks (including the title track) I think they're all in the front left and rear right, which is kind of a crude way to put the element in "quad center" (as Arthur called it) or the middle of the room in other words.
The same goes for the Ship Ahoy sound effects - if you put the stereo recording in the front and left right, and the same recording in the rear left and rear right, you're left with mono pairs along the side walls (ie front left and rear left are a mono pair, and front right and rear right are a mono pair) and the engineers didn't want this. By flipping the stereo image of the sound effects in the rear channels, you create the maximum number of stereo pairs possible - not only do you get two intersecting pairs at "quad center" (FL and RR are one, FR and RL are another), any two speakers in the quad setup also form a stereo pair: FL & FR, RL & RR, FL & RL, FR & RR. So no matter where you are in the room, you're getting a kind of spacious, three-dimensional sound thanks to there being no mono speaker pairs. Would I have done it this way? Probably not, but the options in 1973 were limited, and the SQ mixing rules were constrictive: for example, Arthur told me they adhered to the SQ rule that said reverbs had to emanate from the same speaker as the originating sound, which meant that in the case of this sound effect, they couldn't put the sound effect in the front speakers, and echoes from it in the rear speakers.
As the person who first put forward the original theory that the rears were swapped on this album, sometimes you just have to humble yourself and admit you were wrong, and that you didn't understand the thinking behind the mix decisions, rather than insisting something has to be a certain way because it's the way you would do it. I'd encourage you to do that here.
