Eggs won't hatch while travelling on a boat

Solution 1:

Your eggs not hatching is independent of your mode of transit. This means that whether you are on a boat or not does not affect your eggs hatching. What does affect distance being recorded is connection to the internet, travel speed, and being soft banned.

Connection to the internet - when you visit a lake, often you are going to remote places with no wifi and poor cell coverage. Thus, if the app cannot accurately track your location and connect to Niantic servers, you distance will not be tracked for your eggs. Check your available networks and reassure that you still have ample data coverage on your boat.

Travel speed - it is now confirmed by a large number of players that traveling too fast can affect egg hatching. It is a general consensus among players that the maximum speed for hatching eggs is ~10-12mph (15km/h is about 9.3mph), so perhaps consider slowing down, as you are traveling at just about the maximum speed for tracking egg hatching distance. You can reference this question for more about speed: How fast can I travel?

Soft ban - if you have been recorded for cheating via GPS spoofing or any other form of cheating, you will likely be soft-banned (or banned for a short amount of time). When you are soft banned, you cannot catch Pokemon (they all run away), some distances don't track, and you can't get items from PokeStops. Refer to this question for more about soft bans: Why can't I catch Pokemon in Pokemon Go anymore?

In addition, if it worked yesterday and doesn't work today, it might just be a server issue with Niantic, and will resolve itself in time. It's a bit hard to conclude anything with only 24 hours of evidence, but wait a little longer and try again. If your distance is still not tracking, consider one of the above prognoses.