No one has the correct answer to your question. I grew up water jousting albeit a different style (méthode givordine). Back in the day there was no engine so you needed megn to row the boats. Plus you can’t keep going back to shore to pick up contestants so the extra men are a mix of what used to be rowers and contestants. They also happen to help with boat stability a bit too but not much. I should add that the rowers here are in blue and other contestants in white, clearly.
Methode givordine and lyonnaise are more impressive water jousting, look it up (google ‘joutes nautiques méthode givordine’).
If I may add, water jousting dates back long time ago when poor people did not have horses and still wanted settle arguments. Or so goes the legend. Even today it is a very traditional sport and there are many festivities during that time of the year, whether it is in the south of France like on this video or a little further up the Rhône river, in July and August.