I did this mod a while ago. it really helps with additional air into the engine. As you can see, there is a screw on cap to use when crossing deepish rivers.
http://www.pcoa.org.au/forum/viewtopic.php?f=19&t=4285
Pezzy, that crossing is a very short crossing. With the use of video editting, I cut a photo from the video for the best pic. The pic hasn't been photoshopped or anything, just cut from the video.
If you pull off the inner guard plastics, you can see the pre-air box. There are only a couple of bolts holding it on. Pull it off and take a look at it. It could do with some more diameter inlets and outlets, but it's not bad. Any water has to flow down through the inlet under the guard, up to the top of the pre-air box, then down into the next chamber, then up to the air filter box. It's no sealed snorkel but works well with the addition of a front tarp. (below)
It's a simple lightweight 6ft x 4ft poly tarp held down with shock cord to the underside at the front and down to the lip of each guard. This way it fits R50 with no bullbar, with bullbar, with nudgebar and R51 without bar. Haven't tried any other vehicles yet. Cost about 10 bucks!!
Nick