List each ship with its captains.
select t1.name, t2.name from ship as t1 join captain as t2 on t1.ship_id = t2.ship_id
Which ship has more than one captain?
select t1.name from ship as t1 join captain as t2 on t1.ship_id = t2.ship_id group by t2.ship_id having count(*) > 1