I'm sorry your dog ate your belt. I have two German Shepherds and have lost half of my shoes to them!
I live in Florida and go to the beach and water park all the time. I wear a one-piece bathing suit, which keeps my pouch in place. Since you're a guy, I would think that a fitted beach shirt would work well to provide the kind of support you're looking for. Here's a link to show you an example of what I'm talking about
:
http://www.thebeachdepot.com/Mens-Black-Rash-Guard/M/B000NP0F3O.htm
Where in Hawaii are you going? My dad used to live on Maui, and it was beautiful!
I hope this beach shirt works for you and that you have a wonderful time!
Cecilia