My memory of such things is generally fairly good but spotty. I have to say I have no recollection of a floating drydock from Aurora - or anybody else. I'm not suggesting for an instant that my memory is a definitive arbiter for such discussions. It may well have missed something.
I'd be particularly surprised, though, to learn that such a kit came out in the late seventies. That's when I was working part-time in a hobby shop, and I was keeping up with new kit releases pretty carefully. If I'm not mistaken, it was at about that time that Aurora went out of business.
The bible on Aurora kits is the history of the company by Thomas Graham, which, I believe, contains a complete list of all Aurora kits. I don't have a copy and I think it's out of print; maybe another forum member has one.
I just had a memory flash that just might be relevant. Is it possible that the kit you're talking about was a 1/700 one from Skywave? One of the very first kits that company made was a
Fletcher-class destroyer. It was, of course, a waterline kit. I think it first appeared in about 1977 or 1978. Shortly thereafter Skywave released a "drydock set." It wasn't a floating drydock; it was a set of diorama parts - including one or two underwater hulls to fit the waterline destroyer kits. The box, I think, was roughly square-shaped, and the picture on top was a bow view of a destroyer from the vantage point of the bottom of the dock.
My memory is a weird thing. Sometime in the middle of the night it may recall another kit that fits the description better. At the moment, though, that's the best it can do.
Youth, talent, hard work, and enthusiasm are no match for old age and treachery.