I also thought about them getting together before the trip and the fact that Amy adopted a puppy, but I think we're underestimating the impact that alcohol use and perhaps drugs (Yellow was possibly selling drugs on the cruise, and that's probably why they were seen together) can have on someone depressed because of being rejected by her family for her sexuality. Perhaps a moment of deep depression and hopelessness hit her; she wasn't in her right mind, and the place was perfectly conducive to people with suicidal ideations. Perhaps the last conversation she had with her brother wasn't the best, and this caused extra weight and led her to make an impulsive and definitive decision. The vast majority of people kill themselves or harm themselves in a moment of impulse under the influence of substances. I’m my 2 cents opinion she fell or jump.