***** NOT A TEACHER *****
Juliana,
(1) I think that I have bad news for you: there is no perfect rule.
(2) Generally, the "rule" seems to be: get ON a large one; get IN a small one.
(a) One problem: what is "large" and what is "small"?
(3) I guess most Americans would accept:
(a) Get ON the ship. (Ships are usually large.)
(b) Get IN the canoe or kayak. (They are quite small.)
(4) But I do have some good news for you: sometimes people will not care which preposition you use.
(a) For example, I have just visited the "books" section of Google and discovered that many "good" writers use "IN a lifeboat"
and "ON a lifeboat."
(i) In fact, a very good writer named William S. Burroughs even wrote this:
"It is like being ON a lifeboat. ... Everyone IN the lifeboat is filled with the urgency of the struggle to survive."
(5) My advice: Google "in/on a boat" and study the results. Then decide for yourself which one is the most appropriate.