Newbie question on entering water

[Dragar]

In my most recent game I had pretty much conquered land and was about to head to water. I loaded up my pretender (some kind of undead, forget exactly which) and a Vampire Lord with gear, including a sea goblet, to send them into the watery depths

I was unable to enter water. I offloaded their troops, still no. Tried them individually, no luck. Ended up starting a new game at that point, but would like to know for the future

Is there a limitation on undead entering the sea? Could it be something else? Perhaps sea goblet doesn't work on nonliving??