Saemon Havarian, who also goes by the anagrammatic alias called 'Esamon,' is a corrupt sailor who likes to get other people to do his work for him and put the blame of his wrongs on others.
Regardless of whether you sided with Bodhi or Aran Linvail, Saemon will sail you to Brynnlaw in an attempt to rescue Imoen from the Cowled Wizards. If you sided with Aran he will betray you by summoning three of Bodhi's best Vampires to attack you.
After you have escaped the Spellhold Dungeon, Saemon will appear again. He will advise you to free all of Irenicus' test subjects to aid you in your battle against their captor. This advice needs to be taken as you cannot defeat Irenicus without them.
After Irenicus has fled, Saemon will offer to sail you home. If you reject you have to go through the portal into the Underdark. If you accept he will say he doesn't actually have a ship and you need to steal the pirate's ship. On your journey home you will be assaulted and taken to a Sahuagin city, while Saemon leaves you behind and swims for safety. Either way you will end up in the Underdark.
In Throne of Bhaal Saemon will be smuggling in Amkethran. When you first arrive you will see him he is in trouble with a few mercenaries, being accused of stealing their items. Once Saemon sees you he will shift the blame on you, causing the mercenaries to attack you and Saemon to flee.
After defeating Abazigal and Sendai, Saemon, still in Amkethran in the smugglers' cave, will offer to help you gain entry into Balthazar's monastery. He will disguise himself as someone who is trying to cleanse the Bhaalspawn taint and get you through the gates. Saemon then meets a monk who is glad that he has brought Gorion's Ward to Balthazar and expects the ward to be drugged so no trouble would be caused, though Saemon says that he didn't drug you and felt he owed you one. He will then flee while you have to fight the monk and his mercenaries.