Great replies already but I'll add/stress just a couple more.

Don't take your running shoes, the Birkenstocks will be great for everyday wear, and wear your khakis and dress shoes on the plane so you don't have to pack them.

As already mentioned you can hire a snorkel and masks but they'll almost certainly be poor quality ones. So even though your own can be awkward to pack at least they're not really heavy - I'd take them if you expect to spend more than one day in the water.

And I've no idea where to get one but see if you can get one of those over-the-shoulder straps that are designed to carry the standard size 1.5L bottle of water. You'll need to carry water around with you but in your day-bag is potentially risky re leaks and it's better to also keep both your hands free when out and about. If you can't find one but know anyone who sews, make one before you leave. It need only be like a one inch wide strip that folds up, and needs also to be strong enough to last just 10 days :-)

Finally, if you're a fussy sleeper, take a sleeping sheet with you for those nights you're going to be sleeping rough(er).