Grew up in the sixties in southern California. I have never heard anyone but my family call them go-aheads. Either it was a micro-regional thing that died quickly, or it was just us. Part of joining the wider world was learning to call them flip-flops.
It was the product in Skywatcher’s second link that we were using and calling that.
Shower shoes are Skywatcher’s first link. Flip flops are what you are talking about. Beach shoes are what would be sold as water shoes.
My mom used to call them that. She spent her early childhood (in the late 50s/early 60s) in southern California.
^ This.
That’s what we called them in Japan, and that’s what we called them when I was growing up in San Diego. ‘Flip-flops’ was what everyone else called them.
Incidentally, my new zoris arrived last week. Still wearing the old ones though. Just wanted replacements ready.
I always called them flip-flops or thongs. My grandmother is the only person I know who called them zories.
Long Island 1960s: Thongs as the most common name, with flip-flops a close second. Some people called them zories, but there weren’t all that common.
In the 60s they were referred to as;
jap slaps
gook boots
flip flops
go-aheads
Of course, most those names are not at all acceptable today, but back then, those terms were not uncommon in the California and Hawaii beach crowd.
But Zori are made of straw and cloth, so are not made for water-type applications. And people usually wear tabi socks with them. So other than the basic shape, they’re not much like flip flops.
I grew up in Oregon in the 50’s and 60’s and we always called those cheap rubbery ones thongs. The other meaning of thong was not something we ever saw in those days. I only started calling them flip flops much later, in California.
Never ceases to amaze me.
Here I thought thongs were an Australianism. But from what everyone is saying it seems once upon a time it was a well excepted term in the US. By the way thongs is still the go to term for them in Australia. Differentiated from the underwear by the plural, you wear thongs on your feet you wear a thong as underwear. Although I more often hear the underwear called a G-String rather than a thong.
Thongs. Midwest US, 1960s-70s. Never heard flip-flops until at least 1980.
Flip-flops. Although I have a hard time remembering any of us having any of them. I believe Beloved Brother did at one time but only for a year or two.
The stores called them zories, but we called them thongs. NYC 1960’s.
Ditto
We lived in Japan when I was little, and we called them zoris. So we called the cheap rubber ones zoris as well.
Margaritaville was released early in 1977.
I’m 35. We called them flip-flops, maybe sandals as a catchall term. Most girls I knew as a teen and beyond wore the other type of thongs. Never heard thongs as a term for flip flops until adulthood. Grew up on the beach in the Mid-Atlantic.
Jandals in New Zealand. I believe that’s from “jelly sandals” or maybe “japanese sandals”.
Flip-flops. We’d get a new pair at the beginning of every summer, and they never seemed to last until we went back to school. I’m wearing a pair at this very moment, altho they’re a bit classier. The soles are the same foam rubber, but the straps are fabric and they’re fused to the sole somehow rather than poking thru the bottom with those button ends. Like I said - classy! 
Chicago in the late fifties early sixties - thongs
Hawaiian Dress Shoes
An image search for thongs disagrees. NSFW: enter thongs and search for images.
But an image search for flip flops, now that is SFW and it yields the desired: thongs - Google Search
For me, growing up in Upstate NY they were flip flops. When I went to bootcamp they were called shower shoes.