That would be like "They arrived TO Cuba by boat," which is a mistake users from some language backgrounds make in English. "Arrive" is not talking about the movement into Cuba (movement along a path entering a space) but the termination of the movement, which happens in only one place, entirely within Cuba.
Of course, some languages may use a word like "arrive" differently, but this is also how it works in English and German. German even uses the accusative to show direction like Esperanto does.
English: I go TO SCHOOL [direction] - I arrive AT SCHOOL [location] - I study AT SCHOOL [location] German: Ich gehe IN DIE SCHULE [Accusative - direction] - ich komme IN DER SCHULE an [Dative - location] - Ich lerne IN DER SCHULE [Dative - location]
Great clarification to that natural and excellent question.
Consistent with that answer. We could also say: "La sinjorino alvenis Kubon". This time the accusative does not express movement, but the fact that Kubo-n is the direct complement of alvenis in that sentence.
La sinjorino alvenis Kubon = La sinjorino venis al Kubo?
I have been convinced by Great User Name. The action of arriving happens only in the place of arrival. You can do many things from one place to another (salti, danci, kuri, ktp), but the beginning and the end of the arrival action is at the same place - there would thus be no movement accusative in this case.
The question is excellent - I was wondering myself. I had the same impression as Hakanto - gave me the occasion of seeing the Great User Name answer.
An example with "en" where the movement accusative can be used - salti.
La muso saltas en la liton - the mouse is jumping from a place other than the bed to a place on the bed.
La muso saltas en la lito - the mouse is jumping from a place in the bed to another place in the bed.
As you mentioned, « en » indicates a place, not movement by itself – which is why the movement accusative is necessary to indicate the movement where applicable. In “La muso saltas al la lito”, the preposition “al”, indicating a direction (as you say), already permits to know that the mouse is jumping to the bed from another place – in which case, the movement accusative would be moot.
Both forms are correct. However, the adverbial form is much more used in Esperanto than in English, even though the use of a preposition in Esperanto is also correct.
In the usage, we would see often things like: Kiel vi alvenis? Mi alvenis buse kaj li alvenis trejne.
For example, on another mode, La servo okazas vendrede. The service takes place on Fridays ("Fridayingly").
This is probably not the place to discuss general grammar extensively. I would parse the basic sentence as follows:
Subject (They) verb-intransitive (arrived) adverb-location (in Cuba) adverb-manner (by boat).
For me, the difference between "boate" and "per boato" in Esperanto reminds me a bit of the difference between "me" and "to me" in English in the phrases "give me the book" and "give the book to me."
Unfortunately, my eyes aren't so good anymore: they tell me that "boato" is an at-in-law (like one of those duck-rabbit or old hag-beautiful young lady optical illusions)?!