Tampon help please!!!!!?

Tommorrow I'm going to a water park and I have my period so I need to wear a tampon. Lately everytime I put in a tampon and go into the water it falls out! I am pretty sure that's not supposed to be happening! Anyway I noticed that the tampon isn't really in all the way like it shows a little bit, I don't know what I am doing wrong isn't the only thing supp used to be showing is the string I am starting to think that my body just can't keep a tampon in ughh please give me some tips or tell me what I am doing wrong! Thank you.