It depends more on storms and tides than time of year, I bet. One of the bad jellyfish times was the same weekend a hurricane was moving inland toward the Gulf Coast. Somehow, we hadn't heard about it and only realized what was going on when we were turned away from the park at the Padre Island National Seashore entrance station. Felt pretty dumb at the time, since we then realized "So THAT'S what those weird banded rain clouds are -- the initial rain bands of a tropical storm!"
Justin (OutdoorPhoto.com) and I have visited Padre Island NS and other points along the Texas coast at all times of year -- spring, summer, winter, fall. Can't recall the last time we saw the jellies on the beach or in the water, but they are definitely not rare. One can call the National Seashore information line and ask the rangers for a seaweed and jellyfish status -- the same status they post on the chalkboard at the entrance station to PINS. Seaweed can be as much of a pain as the jellyfish. Always hated feeling that stuff get wrapped up around my ankles -- your mind plays tricks into thinking it's something else entirely! (Not a shark, mind you, but there are lots of living critters in the water most of us wouldn't especially like to have attached to our legs!
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