You would "get into" a small boat (e.g. a rowing boat), but you would "get on" a big boat (e.g. a yacht).
Here are some examples of big things we "get on":
Get on the bus/plane/train/(big)boat/ferry/hovercraft/motorbike (we sit "on" a motorbike).
However I'm not sure if this is a rule.
I'm not a teacher.
Student or Learner