My parent’s small pond, which has never seen a boat or any other real human activity, got them before the big lake it’s connected to did. Clearly there was some other way they could spread, presumably by bird.
Anyways, one by one every lake in the area no has zebra mussels. Even if they would only spread via human, it was clearly only a matter of time. As much as they suck (they’re sharp and attach themselves to basically anything in the lake) I’m not sure the expense has been worth simply delaying the inevitable.