It's the simplest conclusion from the script to think they're the same lake. Sure there's a possibility for a second lake, but if I talk about going to "the treehouse, I was always there with Nami" it's a singular place. Plus Nami ties the lake to Summer, by specifically trying to arouse Nika's thoughts about Summer at a place where he has memories of her. The scene is weaker if it's just a random lake that Nika has no prior memories about. The whole scene is about nostalgia, with Nami trying to get Nika to remember better times, and Nika actually remembering things about being younger. We also know they shared their cool lake spot with Summer from the flashbacks, and it was probably more than once - that was the whole point of Nami wearing her hair provocatively.
The circumstantial evidence, the language used etc all point towards it currently being an oversight. It's far more likely (currently) that it's the same lake and Ocean will either ignore the math, and just hope people don't care, or retcon a whole bunch of stuff for it to make sense. It's also possible Ocean will explain it all very sensibly by adding info that we don't currently have, all the bridging info that helps make sense of how the cabin is so ridiculously far away. But it's really only one render and a crossfade away in an AVN anyway.
Everyone's free to have their own opinion, of course. I'm not going ot keep debating it today though, I'm just repeating what's already been said.
