I am no where near 15 but I can remember how hard it could be to meet new people.
I am an only child so I never had any companions my age traveling.
I could hang out with adults and get treated like a servant or I could go find kids to hang out with.
After many struggles with awkward moments I find one thing always true, people love to talk about themselves. A smile and a friendly question about the person will normally kick things right off.
I am in my mid 40s so I have no teen relavent advice, as my 15 year old daughter will you
Have fun on your cruise.