Because the question was really: If you had adjusted the save based on the sailor's knowledge of the seas and the certainty of death, would they have then passed?
I'm not really understanding your question. From my perspective, everyone trained in armor (presumably this captain is) would know that swimming in armor is life threatening (i.e. "or if your command is directly harmful to it"). Now, the OP did not tell us whether this was at dock, out on the ocean, or in the middle of a river. So we do not know how threatening this is. And yes, I understand that some DMs might read that phrase as "hit point damage".
But from my POV, there is no need to adjust the save. There is no need for a save unless we are talking certain specific conditions (like docked in shallow water where a person could more easily just go to shore). In most circumstances, this should be an auto-save because it is directly harmful.
I believe Coredump's question was in regards to this, not that failing a save means failing a save. I'm not sure if there is a rule in 5E adjudicating how often a target gets to attempt a re-save when the risk of death is high. In older editions you got more saves more often with lower DCs.
Command isn't supposed to be run like Dominate Person.
I agree, but I don't think that the OP's example is a good example of how Command might need to be adjusted. Command is fairly explicit. No need for rerolls for undead. No need for rerolls for a creature that does not understand the caster. No need for rerolls for directly harmful commands. The target just auto-saves in these cases.
Generally speaking, 5E lacks granularity. The system is very black and white. Save/fail. Advantage/disadvantage, etc... This situation calls for more than that because the precedent it sets is for a supremely buffed Command and supremely nerfed scenarios where NPCs are incapable of fighting back.
This situation doesn't call for more granularity. This situation requires an autosave.
Now, I'm sure that people can come up with a lot of situations that might require granularity and it's fine for a DM to give a +2 bonus or penalty or even advantage or disadvantage against Command in those cases. Personally, I'm not that type of DM that does that and I don't think this example comes even close to being one for which granularity should be considered. But, I could see other DMs doing it.
