Well, they are venomous, but not quite that venomous. Depending on the person stung, they might have a rather mild reaction, or be paralyzed in a few minutes. The big problem, as I mentioned before, is that being paralyzed under water is a very bad thing. Either way, if you see a cone shaped shell while SCUBA diving, don't pick it up.

As for the Chuuls, while there might be other large arthropods living in the D&D world, the Chuuls were created, so they don't fit into any ecological niche that even a giant crab may fit into in the crazy D&D world. That, and if I remember correctly, they are rather evil and cruel, rather than just living by instinct like a giant crab.