70,000 sounds like a lot to be active on the lake on a normal weekend. That would be about a third of all boats here at the lake being out, there are a lot of multiple boat and watercraft owners. I just can't imagine it is that high. But I am sure someone counted them.
That may be the number of "sorties" over a weekend including people that take a half hour trip or something.
There are also a lot of boats here registered out of state like IL, KS and IA. People register at home rather than where the boat is used to most so the actual number of boats kept at the lake is probably higher