Where the heck are you walking? No gunk in mine (yep, I just checked)!
I've been on the beach with them, as well as in the ocean with them (I can't stand the feel of the ocean floor).
Here's a story from South Carolina. We went beach combing to a private island. We do this at low tide and get there by boat with our friend Scott. Scott pulls the boat up as close to the shore as he can get and then we get off the boat and walk to the land. The kids are rearing to go the minute he stops the boat and have no problem getting off. They do, Gary gets off and then it's my turn. Remember, I don't like the feel of the ocean floor AND I don't go in the ocean as a rule, so I step off the ladder, onto the bottom of the ocean and it's muck! Muck!!

My foot, with my croc on it to protect me from feeling the ocean floor starts to sink.

So, I'm not sinking into this gross muck that I can't stand the feel of. The more I tried to get my foot out, the deeper it went. I had no choice to to pull my foot out and then, my other foot started to sink.

So, now I had to take my feet out of the shoes and stand there. Scott went into the water to retrieve my sandles off the ocean floor. What a sport he was.
Even funnier, later as I'm walking along the beach, I took my shoes off and left them behind. Scott reminded me a couple of times not to forget they were there and even moved them closer to the boat for me. I guess the idea of me not walking back out to the boat without them on my feet was enough to scare him.
Next year, I think I'm getting a pair of croc boots to wear when we go beach combing!