Zoom in a bit west and a little north of the Cayman Islands. There are some runners and some bikers in the ocean. I'd guess these were people on a boat.

Zoom in to Puget Sound and you can see the same thing, clearly showing the ferry routes. One oddity is that the density of bikes seems a lot higher in the water near the Bainbridge Island docks than in the water near the Seattle docks. All of the ferry routes to/from Bainbridge are from/to Seattle, so at first I'd expect Seattle to be at least as dense as Bainbridge, because it has all the Bainbridge traffic plus Bremerton traffic.

I wonder if what is happening is that most of these people live on the Bainbridge side, and turn on their tracker when they take their bike to the ferry to head for work in Seattle. Then they turn it off on the ferry to save power. They do not turn it back on until the ferry docks and they are leaving. Hence, for the morning commute, the only dots in water are on the Bainbridge side.

For the even return to home, perhaps many whose work is only a short ride from the docks do not bother to turn it on to track that segment. They only turn it on when the ferry gets to Bainbridge. Hence, on the Seattle side of the Bainbridge run, there are mostly only dots from people whose have a significant ride on the Seattle side to get to the dock.

Anyone have a simpler explanation for the Seattle/Bainbridge dot imbalance?

I was a bit surprised at how much running and biking there were up in hills and mountains, such as north of Los Angeles.

Also I was a bit surprised by how little running there was in Jamaica.

This is based on data provided by an app. I couldn't find Jamaica on any list of countries by smartphone penetration.

This showed me that little Winder, Georgia isn't as unfriendly to cycling and running as I thought.