The first question: "por mar" is an expression used to refer actions related with sailing. Not only transportation but also traveling can be used with this expression, for example: "me gusta viajar por mar"; sometimes it's more accurate to say "navegar", "ir en crucero", etc... The second question: in this context the verb "pasar" is not reflexive, thus you must not write "se". "pasar" has some different meanings in spanish, in this case it means "happen" or "occur", in other situations you need to write it in the reflexive form, let's see some examples: "Me he pasado toda la tarde jugando". Why is "me" here? if you look closely the meaning of "pasar" here is "spend" and the action is applied to a person "No es bueno pasarse con las drogas". (As you might know the reflexive pronoun can be placed right after the verb in some cases all toghether.) "se", in this case the third person is used to generalize the sentence. "pasarse" (this form of verb is always reflexive) means to exceed the limits in a specific situation (taking too many drugs/alcohol, having a bad attitude, etc) and this verb also describes the action of an individual. So, from what you have seen so far, you can make a simple rule (which might not be always right, but will work for most of the situations). If the verb is NOT describing the actions of a person or a living thing, you must NOT use reflexive pronouns (do not forget that, as in every language, there are exceptions). So now, what is the verb "pasar" describing in the sentence you wrote? Do you get it now? : )

Por mar: se refiere a vía marítima, un transporte que se hizo vía marítima, en una embarcación. La segunda oración está correcta, volver a ocurrir. Aunque la forma con se también es usada. Volveré a responder tus dudas.