My son was Edge age before the app was reliable for texting so what we did was leave a dry erase board on the door of our cabin (or in the cabin would work, too).
Strict instructions to not go in anyone else's room and not to let anyone into ours. Otherwise, if he's exploring the ship, no RUNNING and behave. If he was going to hang out somewhere other than Edge (or within sight of Edge) he had to leave me a note (like if he was going to a class or a trivia or a movie or something). If he was going to get ice cream/drink/snack, watch funnel vision, that was okay, but it took a little bit of trial and error to figure that out (if I went looking for him in Edge and he wasn't there, I'd do a quick walk around the deck and if he wasn't there, head down to the room where I'd often find him).
Same for me, I'd let him know via the dry erase board where he'd be able to find me. Honestly, it worked this way more than the reverse.