Luckily, I live about a 25 minute drive from one of the best surfing beaches in Japan. It's open Pacific Ocean all the way down to New Guinea, so the waves can get rather big at times.
. Hard to get shots with all the spray. Luckily, I brought my waterproof cover (lots of blowing sand and water), which seem to work well enough for the 50 bucks I paid for it.
This thing. Actually, it was the first time I used it (bought it, I think, 10 months ago). It covered my RF 100-500 w/1.4 extender no problem. The end around the lens hood stayed fairly secure--just a little slippage--and at the camera end, I tightened it so the side of the eyepiece blocked it on one side with just enough room on the other to slip my hand in. I was thinking I might glue a couple of small rubber door bumpers on the lens hood to prevent any slippage.