I call it "offroad kayaking", though I usually use the term for when you scooch your butt over a shallow bit of river or pull yourself over a mostly submerged log blocking your way.

I'm not a kayaker, but I'm casual American English I've never heard a term for this

Seal launch

"running start" would be the closest thing I could think of in American English, but it's not a perfect match. As it pertains to kayaking, a "running start" would be understood as some sort of momentum transfer while simultaneously hopping into the kayak.

But still, that definition is what a typical American would imagine if you said "kayak running start". So perhaps not totally identical to what I think you're describing.

I haven't heard of a term for that en anglais, but I would call it "scooting in" ou simply just "launching."

"Kayak launch"