They are very similar in meaning. The first one, with "had", sounds like you are explaining something that happened a while ago. You may be speaking to a third person about the cruise. The second sentence sounds more 'in the present'. It would be a direct conversation about meeting the person as if you are hoping they will remember you.

They can both be correct. If you're narrating it purly from the present standpoint, you should say "We once met ...". There is no reason to say "had met". E.g. I am seeing this person tomorrow. We met once on a cruise liner in July of 2019. If you're narration is centered around a past event, like saying you met the same person again in 2020 for instance, then you say "We had met ..." because there are two past events and the encounter on the cruise ship happened earlier. E.g. I ran into this familiar looking person. We had met once on a cruise liner in July of 2019. (It sounds better to me with "a" and "of")