Thanks Dave. What eyepiece viewer did you buy? I'm also thinking of a weight belt. I'll probably try and rent one when I'm there. I don't want to worry about carrying weights in my suitcase. As an alternative I might buy the belt at home and just rent weights there. Now I've got to stop worrying about shallow water blackout - I never even heard about it until last week. It seems like hyperventilating is the main cause, so you can bet that will NOT be a technique I use. From what I'm reading, limiting my time underwater to something reasonable and not pushing my limits will also help. I can hold my breath while sitting for about 90 seconds. Under exertion, I'm guessing maybe half that amount. I'll aim for 5 seconds to dive, 15-20 seconds underwater, and 5 seconds to ascend. It seems reasonable... I've also read that I should relax and take in a normal deep breath before the dive. Apparently oxygen levels won't be that much better by taking 2-3 deep breaths/exhales prior to the dive...
Here's where I got the eyepiece viewer: Photosolve Home > Products > Xtend-a-View
Note than depending on actual location of the buttons on the back your waterproof case, it is possible than none of the viewers will fit. You won't know that until you actually try to fit a viewer on the case. However, it turned out they fit perfectly on the cases for both my cameras. For attaching the viewer to the case, be sure to use industrial strength velcro from a hardware store. The cheapo velcro that comes with the viewer will come unglued after just a few days use in the water.
Renting a weight belt is easy in any location that has a dive shop. Cost is about $3/day.
You have been reading way too many of these freediving posts from competition-driven people concerned about achieving their "personal best" instead of enjoying and observing the underwater world. Relax, take it easy, you don't have to worry about how long you can hold your breath or if you will get shallow water blackout. Snorkeling is not technical, and that is one of its joys. I never think about shallow water blackout because I'm never holding my breath anywhere near that long.