I don’t know about in a kayak, but I imagine that the current is too strong for a swimmer to hold on. The water will also suck you down deep underneath, almost certainly lodging you into one of the tunnels for the short remainder of your life.
There's a few spots near me that are well known danger spots for kayakers due to this phenomenon.
> The Strid (grade 5-) is a long thin section of river where the Wharfe narrows to 5-6 feet in places. The main problems are beneath surface in form of ledges which if you're swept under - you can forget it! At a higher level as seen in pictures, it's runnable. The pictures show med-high water and most of the ledges were covered. I'd have graded it at 5- this day. At lower levels it runs at class 5+ and many would portage, the ledges that trap you need divers to get to you and even good safety cover would be of little use in an entrapment situation. (2 tourists fell in at the Strid last year and one didn't emerge for nearly a week)
Portage in this context means get out and carry your kayak past it. From https://www.ukriversguidebook.co.uk/rivers/england/north-eas...
You're not coming out alive.