There will be people of all sizes - don't worry about it. Everyone is there to have a good time, and you should too. Having more body fat than my husband makes it easier for me to float comfortably! You have to get in the water! There's nothing that compares to swimming, floating, snorkeling in the warm ocean.
For the ruins, I'd pack some comfortable high top boots or shoes that give good ankle support and have good traction type soles.
Enjoy your trip, and post a report when you get back - for us who haven't been on this cruise yet.