This is kind of European English. Nobody knows what it really means.
So we need reasoning here:

Seaworthy packaging implies very sturdy packaging, probably wooden boxes or crates or something like that. A cardboard carton (картонная коробка) doesn't sound sturdy enough for a seaworthy container. So it is probably out. But we still don't know what type of container it is, so it is safer to use something very general -- ящик/бокс для морской транспортировки (it might be a wooden crate, a solid box or whatever).

A multiunit container sounds like тара с отсеками. A possible translation: многосекционная коробка/бокс.

Personally, I would write бокс everywhere. Not clear? Then they can request photos of all those containers. I don't want to call something ящик so that people can later tell me it was a коробка or vice versa.