芋(の子）を洗うよう

like washing (a bucketful of )potatoes:
So crowded you can hardly turn around, jam-packed, mobbed with people.

Summer weekends at the beach in Japan are impossible. The beaches are so crowded that you can hardly make space for your beach mat. When people play n the waist-deep water, wave after wave jostles them into each other. This commotion resembles a wooden bucketful of potatoes sloshing around while being washed. Usage is restricted to water-related scenes.

七月下旬の日曜日、夏休みに入ったので、子供連れの家族が多く、海水浴所は芋（の子）を洗うような混雑ぶりとなった。

Since it was a Sunday toward the end of July, where summer vacation starts, there were a lot of families with kids, and the beach was jam-packed.