The historic rowing race was brought to a standstill at the halfway point of the 6.8km course and there was a 31-minute delay before it could be restarted.
British authorities arrested Australian expat Trenton Oldfield, 35, who claimed responsibility for the stunt on a blog entitled "Elitism Leads To Tyranny".
Oldfield, in a wetsuit, narrowly avoided being hit by oars when he swam into the path of the boats.
"THIS IS 'PEACEFUL'... I HAVE NO WEAPONS (DON'T SHOOT!) MY ONLY FEAR, IS NOT SWIMMING FAST ENOUGH TO GET IN THE RIGHT POSITION TO PREVENT THE BOATS," he wrote on his blog.
"My swim into the pathway of the two boats today (I hope) is a result of key guerrilla tactics; local knowledge, ambush, surprise, mobility and speed, detailed information and decisiveness."
Oldfield wrote the aim of the tactics was to "shift from being a victim".
"To being the ones setting the agenda, placing elites more and more on the back foot, increasing their costs, causing confusion, fermenting internal mistrust, creating embarrassment (a Tory's worst nightmare?), frustration and manifesting a vulnerability."
He is facing public order charges.
It's believed Oldfield studied contemporary urbanism at the London School of Economics.
Another Australian, David Nelson from Brisbane captained the Cambridge team to victory.
"We've said all the way through the campaign to expect the unexpected," Mr Nelson told The Guardian.
"But we didn't expect this."
The Oxford boat club president Karl Hudspith took to Twitter to voice his anger at Oldfield.
"My team went through seven months of hell. This was the culmination of our careers and you took it from us," he said.
