I just caught up with some routed sea raiders for the first time ever.
They said "Please, Leave us alone" and when I told them they would be my prisoners or die they replied
"We have no strength left to fight, we surrender to you King Akimba"
I experienced this before, and went to try again against a large number of runners - they started out the same but when I asked them to surrender they rallied and attacked again! So its not a given that they'll surrender if you catch them fleeing.
My favourite is still when one gets away and runs straight into another group of Sea Raiders who attack and capture him.
OT: I've never seen this. Whenever I meet up with a routed enemies party they just *poof* disappear (as in, their whole party simply vanishes immediately)... Weird!