Based on my experience tracking boats it is most likely one of two things:
1) You need to specifically enable backwards flight for active track, the default is off. Otherwise if your MP is next to, or in front of the boat, it will only track with the camera until it is behind the boat, and the boat starts to move away. This is in the Visual Navigations menu (See Below).
2) Active Track has three modes: Trace, Spotlight, and Profile. What you describe is Spotlight mode in Active Track. When you go into Active Track, you can pick which of the three modes on the right of the screen. Make sure you are selected to Trace mode.