As a large plus sized woman (275 lbs) who just got back, let me put your worries to rest. Yes, I look horrible in a bathing suit and I'm pretty self conscious, but I made up my mind early that I was not going to let it stop me. I went to the pool to tan every day (by my self mind you because my friends were not really "sun people."). Yes, I did feel a little out of place next to the teeny tiny bikinis, but let me say that there were a lot of those teeny tiny bikinis that did not contain teeny tiny bodies. I think that there were a lot of people who looked worse than me because I wear a suit appropriate for my body type. I had no trouble with anything - toured the ruins in Belize city (Altun Ha) - you can't climb them anyway, so it was not at all difficult - just flat walking. There was one large one that people could walk up if they wanted (only one person in our group did -t he rest waited on the ground). I snorkelled with no problem - if you book a private tour, they will not make you wear a life jacket if you don't want to, but if you book a ship tour, they will. I did both - private tour for snorkelling and stingray city in grand cayman (used Nativeway watersports and their boats were easy to climb in and out of). Also did a ship tour of snorkelling ( by myself) in Cozumel. Had to wear a life jacket but it is totally adapted to any body type. I know exactly how you feel, but hopefully, you'll be able to relax and do everything you want to do. I did hear that cave tubing in Belize is a bit difficult if you're out of shape, but I didn't look into this since my friend didn't want to do this anyway. My biggest worry was the airline seat (Spirit Airlines) and although we were crowded, it was no major worry (even the seat belt was fine). Please have a great trip!!