I believe "S" is a Bugbear Strangler, who was hidden during the first couple of rounds of the fight. He was probably clinging to the side of the boat waiting for everyone to be distracted with other foes.
I'm pretty sure the yellow cross is the fighter's Mark, which is described on page 76 under the "Combat Challenge" class feature and also on page 277 in the list of conditions.
Flanking is described on page 285. In order to gain combat advantage against the bugbear by flanking it, Jude or Druthruss can move to L10, flanking it with Sahyr and standing over Aridha (see "Moving through occupied squares" on page 283).
Or, Jude could move to M10, Druthruss to K8 to flank, Vatan shift to L10 to flank with Sahyr and stand over Aridha, and Sahyr can stay put and keep combat advantage.
edit: Yes, "S" appears in post 107, vaulting over the rail onto the boat. IG didn't explicitly say it was a bugbear at that time, though.