Don't worry about it as a lot of this is going to be out of your control. The last time I went to the Caribbean I did nothing unusual and my body was sooo bloated and looked so bloated in my pictures. I still have no idea why! The humidity? Heat? Water retention? I felt way better after coming home.
You can just pack a very flowy oversize button up shirt or dress to layer on top of your swimsuit when you're not in the ocean or pool. And a big hat and sunglasses to make your body look smaller
Honestly you should enjoy the family time, and the heightened sense of tropical fruits and drinks, and having an appetite from swimming and being outdoors all day. Your body shape should be the least of your worries. Take a long walk in the morning or after meals helps too.