Aye sailor! That's a very good question.
Polarsteps at sea for iOS users
At the moment, Polarsteps on iOS does not use GPS functionalities. Therefore tracking at sea is not supported. However, it's on our roadmap and we hope to add this in the future for all iPhone users. We do have a nice little workaround for those PROs who can't wait any longer:
Pro Tip: If you turn on a navigation app, such as Google Maps, and use navigation in the background of your device, then Polarsteps can also use these GPS locations to track your route more accurately.
Polarsteps at sea for Android users
Yes, if you set your Travel Tracker mode to "High Accuracy" the Travel Tracker does work at sea. You will find more info on the different Travel Tracker modes here: How does the Travel Tracker work.