and here's the definition of tombstoning:

"tombstone" refers to a situation after a surfer has wipedout and is being held under water for a period of time. Since the surfboard is so buoyant, it will naturally rise to the surface, but since the surfer is still attached to the surfboard via his/her leash (legrope), the surfboard cannot return to its natural horizontal position. Instead, the pulling of the surfer's weight underwater causes the surfboard to "stand" perpendicular to the water's surface.

Art by  TaviMunk