Captain Wayfinder, as his name implies, is the captain of the vessel. He inherited the ship from his mother, who had passed away a year ago, following the events of the Dragon Crisis.
There is also Guthrum, who serves as a navigator on the Sea Squall. He has sailed so many rivers and seas in Skyrim for such a long time, he could navigate them blindfolded by now.
Ravam Verethi is the third crew member of the ship. He is not very fond of Captain Wayfinder, but mostly sticks to the Sea Squall because of the gold. He also points out that he could attempt to murder Wayfinder and take the ship.