For Viking ships. As far as we know, they just went overboard. This is no problem for men, but it is interesting to ponder what the women who fared on these ships did. We knew women travelled on these ships, so what they did, that's an unsolved question for now. Most reconstructions of viking ships have had portable toilets or a potty, but this is probably not historically accurate. As far as we know from Archeological evidence (oseberga best example), there were no toilets on the Viking ships, so the most likely guess here is that they went overboard, and some may of used a bucket. Unsurprisingly, what they did when nature called isn't mentioned in the sagas much.