Stuff they could've done

What they should've done is use Daniel's knife to reflect light immediately upon surfacing and noticing the boats so far away. The sun was shining brightly and the knife was reflective, but the boats could've seen it. Daniel attempted to reflect light with his mask when they had drifted far out in the shipping lanes, but that's not nearly as effective as the knife would've been.

There's no way they could've been noticed otherwise. That knife was their salvation, but it had to be thought of to be used from the very start, of which they didn't. It would've made them stick out like a sore thumb.

Thinking about this movie and how i would have reacted, apart from freaking and suffering a nervous breakdown, some points do come up:

1) whether swimming was any option at all. In the whole movie, they saw nearby boats, a huge oil tanker and nearby buoy. Of course these were really far! A non static object would certainly have been a waste of time and energy, but a static buoy would have possible, even if it took them a day. These guys had time, just not luck, as the shark got to them first.

2) This is in hindsight I know, but in this day and age i wonder whether these diving incidents occur enough time for even tourists to carry precautionary equipment like flares, sat equipment etc.

3) Again this is more about the likelihood of the event. It is astounding to me, even for a short tour or excursion (and i do these a lot) and large group of unknown people, for a couple to be so isolated as to not talk to a single other person on board. I guess i have seen this before, and they are usually couples completely in their own world, who won't even say "hi"