Which of the following is NOT a flotation device found on United's aircraft?

In the context of flotation devices used on United Airlines aircraft, the correct answer is rescue boats. United's aircraft are equipped with slides, ramp slides, and slide/rafts, all of which serve specific purposes for evacuation and safety in emergency situations.

Slides are inflatable devices that deploy from doors to facilitate a quick exit from the aircraft. Ramp slides are similar, designed for use in situations where evacuation needs to occur on a slope or ramp, ensuring safe descent for passengers. Slide/rafts combine the functions of slides and life rafts, providing an inflatable slide for evacuation while also serving as a flotation device once in the water.

Rescue boats, on the other hand, are not standard equipment on commercial passenger aircraft like those operated by United. While they may be utilized in specific maritime rescue operations or found on ships, aircraft generally rely on the aforementioned flotation devices for emergency evacuations involving water landings. Therefore, rescue boats do not align with the safety equipment typically found on United's aircraft.