The answer to that question is “some do, but not all”. Cruise lines compare their ships to hotels and resorts, where signs often warn that no lifeguard is on duty and guests swim at their own risk. I doubt anyone would disagree that it is the responsibility of the parent to assume the first layer of supervision, but is that realistic on ships that may carry as many as 1,000 children on a summer voyage?
Disney became the first major cruise line to hire lifeguards for its family pools in 2013. The lifeguards are always on duty, so long as the family pool is open. He /she is constantly moving around the pool and there are even multiple lifeguards when necessary. This move came after a tragic accident earlier that same year.