One more thing:
I addition to an alarm clock, I also bring a kitchen timer with me on my cruise. So, that way, I could take a nap and wake up exactly 30 minutes, 45 minutes, or an hour later. That way I would not miss a planed activity because I overslept with my naps. During my cruises, I often nap allot.
And just like my alarm clock, I found the smallest possible kitchen timer.