(haven't seen it yet, BUT...)
why didnt they all fall out of the boat/vehicle after being dumped by THREE HUGE WATERFALLS?
the same reason that in "temple of doom" they were able to safely land an inflatable raft that inflated itself as it fell thousands of feet from an airplane onto a snowy mountain and THEN navigate said raft off a cliff into a raging whitewater river another thousand feet below. all without a scratch.
AND
before that when they landed in water in the tunnels, the trader guy lights a torch with what? a wet lighter

the same reason they were able to wade through the gasoline in the tunnels below the library while holding a torch dropping flames into said gasoline and THEN evade consumption by fire when the bad guy ignites the gas by SUBMERGING THEMSELVES IN IT in ""the last crusade"
the reason being, of course, IT'S FRIGGIN INDIANA JONES, right? which is why i recommended watching this one with a grain of salt and from a 10 year old's perspective. (by the way, how did your boys like it?) i've heard some good things and bad things. most of the complaints i've heard though aren't really plot/development complaints, but bad CGI and the like. which after the star wars fiasco, i fully expected. i'm kinda looking forward to seeing it. maybe even tonight, since i've got the night off.