I would make sure that your 100mm lens + 500D diopter will fit in your port without scratching the inside of the port glass.
the best way is to load the camera with the lens focused to its mimimum (longest physical lenght) if the lens is not internal focusing and the diopter in the housing, put a lens cap on the diopter, cover it with sufficient lens cleaning tissue, put the extension first, then gently and I mean gently deposit the macro port on top of the macro/diopter/lens cap/lens tissue, you should not have any contact with the tissue and the lens of the macro port AT ALL, you need to have a "breathing" space between the serrated edge of the diopter lens and the internal surface of you macro port.
Have a good trip and don't stress out on that stuff too much, just stay focused and pay attention on what you do while setting up and you'll be fine. keep an eye on your gear and who is handling it while entering the water, making sure they are holding it by the grips and not by the dome, strobe arm or accessory view finder, same goes for the rinse tank, these are the accident zones, so be extra cautious in those situations.