The Kattegat is a small body of salty water connecting the North Sea with the Baltic Sea. Both are called Sea for a reason as they are too small and in case of the Baltic too shallow to be anything remotely resembling an ocean.
The Kattegat is roughly 200 km long and 100km wide and therefore not even a sea. It is just an isthmus between Sweden and Denmark connecting the both seas. The connection to the North Sea is via a sharp bend and the detour through the Skagerrak, the connection to the Baltic Sea runs through two narrow fjord-like passages on both sides of the island of SjŠlland.
Swells from the Baltic Sea, if they even exist, have no chance to make it into the Kattegat. Swells from the North Sea have to make it through the Skagerrak and need to bend for about 100 degrees to enter the Kattegat. Local winds have a fetch of maximum 200 kilometres to create waves in the Kattegat. Do your maths and calculate the chance for swells running in this area. Some low period wind agitated ripples will be the maximum what optimistic guessing will foresee.
So, what┤s the point for all the Swedes and Danes driving around with often tiny surf boards on top of their cars? What┤s the point for people coming as far as from Stockholm, Norway or even North Germany into this area looking for waves? What makes them act that folly?