Is it permissible for someone who missed the Maghrib and 'Isha prayers from the previous day to combine and shorten them behind a congregation performing the current prayer on a ship?

What you did, making up the prayer when you remembered it, is correct, and you are not asked to do more than that, for the Prophet, peace and blessings be upon him, said: "Whoever forgets a prayer, let him pray it when he remembers it; there is no expiation for it except that." You are not required to repeat what you prayed before making up those forgotten prayers, because the order is dropped due to forgetfulness. Making up a prayer behind someone who is praying a current prayer is a matter of scholarly disagreement, but the fatwa we follow is that it is valid. Making up the Maghrib prayer behind someone praying Isha is valid, and praying Isha behind someone praying Maghrib is valid, provided the prayer is completed and not shortened.