Today’s three biggest cruise lines—Royal Caribbean International, Norwegian Cruise Line, and Carnival Cruise Lines—share a lot in similarities.

All three lines feature spectacular onboard amenities, from towering waterslides to sophisticated rock-climbing walls to world-class fitness centers. They boast some of the finest dining programs around, with internationally renowned chefs, as well as dazzling live entertainment, and extravagant parties. The cruise lines treat their passengers to cushy rooms, lavish spa services, and cutting-edge enrichment programs.

But for all their luxuries, glitz, and glamour, not a single one of the three most popular and prosperous cruise lines of today have lifeguards onboard …