I was in the NT last year and everywhere I went there were signs, and plenty of references in the literature (travel guides, brochures, maps etc.) all stating that you must be very careful where you swim, only swim where you are told it is safe to (i.e. signs that say it's OK), ask local people, and if in any doubt whatsoever - DON'T!
If a tour guide told tourists it was OK to swim in some area that is occupied by salties, then there is a problem with the tour guide industry. Especially if they let any old idiot become a guide.
While I was in the Mataranka thermal pools a tour group rocked up, mainly young (20 - 26 yr old) tourists from Europe and the States (and mainly gorgeous young ladies). The Tour guide was about 25, and he seemed to enjoy being in a position where the girls hung off his every word.
Above the pools are lots of large Golden Orb Weaver (Nephila sp.) webs. The tour guide warned his charges about the spider webs, but said that the spiders stayed in their webs and never bothered anyone. A British girl asked if they were dangerous, and he replied "This is Australia - all spiders are deadly. :shock: Don't get bitten, because the nearest hospital is in Darwin." For those that don't know, Darwin is several hours away from Mataranka.
He then reminded everyone that the spiders remain in their webs. I was ignoring him at this point, 'coz he wasn't actually doing any harm. Someone else noticed some spider webs in the vegetation at the sides of the pool, webs spun by some Long-Jawed spiders and a variety of St Andrews Cross Spider. The tour guide told them that these were babies (or males), and were just as dangerous as the others, but they tend to stay in their webs or hide, and weren't a problem.
At this point, I opened my mouth and corrected him, trying not to make him look like a complete idiot (just a partial idiot). When I had finished he was still smiling, but it was a forced smile. Ten minutes later he pulled the group out of the pools saying that had a "long drive ahead" and had to make a start.
Sorry for the long post, but I hate dickheads who won't admit they are dickheads. And this guy was up there with the best of them. Worst of them. Whichever.
Hix