فَغَشِيَهُمْ مِنَ الْيَمِّ مَا غَشِيَهُمْ
Then there overtook them from the sea what overtook them.
When a relative expression refers to a tremendous or terrifying event without spelling out all its details, the unresolved magnitude can intensify awe. The verse lets the impact of the overwhelming sea remain powerfully open.
The same general structure can enhance or disparage a referent. Whether the effect is awe, praise, or contempt depends on the content of the clause and the overall context.