Why do this? Just so he can be a nuisance and get into the news? The desperation to get 3 minutes of media attention must be overpowering.
If the rowers were not alert or did not care to avoid smashing him with the oars, he's badly injured or dead.
Boat Race 2012 man in water Oxford vs Cambridge - YouTube

Or was it a ploy that was plotted beforehand to spoil the rowing match when a particular team was clearly losing.

The organisers should have called off the whole match and postponed it to another day. Or if the swimmer was later found to be from either Cambridge or Oxford, then that team will forfeit the match and be deemed the loser.

The problem is that when people do this to get their 3 minutes of desperate fame; they only get a slap on the wrist or 10 fine. They do this because they expect to get off lightly.