Hell, I've had this happen out in the open ocean, while swimming a 1/2 mile off shore... No lanes, no markers, no "path" to speak of, yet two, heads down swimmers making their way in and out of a cove... and blammo! You lock arms in a most painful way... crash heads and wonder... "where the hell did you come from... "

Now things are a bit different out there in open water... you generally are not looking ahead but occasionally, you tend to navigate by watching fixed points on land when you turn your head to breath... and even if you did look ahead, there is a good chance anyone else ahead, and low in the water, is obscured by the rise and fall of the ocean waves.

But still, in all that water... you really do not expect to encounter someone... yet... blammo! Oh if only there were a line or something...