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.

As far as I have experienced the only order of eggs is whether or not they're currently incubated (and their chronological order), since they're on top of the list. The rest is most likely random.
The (non-incubating egg's) order might even change when you pick another egg to incubate or get one to fill an open slot. This leaves only two possibilities:

• Random order
• Fixed order, but this means the setting determining the sorting order changes when you start a new incubation (contained Pokémon for example) ...which is very unlikely.
  This would mean, if they're sorted by ID the contained Pokémon changes every time the amount of eggs changes. Or the IV, or Level, etc. To be honest, that would be stupid programming since they'd be trying to randomize the order and could simply use a pure randomizing algorithm instead.