The monsoon weather seems to bring them ashore. They can be a big problem if you like to swim. In calm water you can see them but when it's rough they wash up on you. I'd say they are around from June- September.
I have not seen any deadly creatures so far this year. Not on the beach anyway! We jetski every week end and I have come across a lot of sting rays in cholla bay but they pretty much stay clear of you.