Trenton Oldfield swam in front of the Oxford and Cambridge crews at the halfway point of the annual race in April, narrowly avoiding being injured by the blades of the oars.
Oldfield, who was arrested for causing a public nuisance, said at the time that his stunt had been to highlight what he perceived as elitism in the United Kingdom.
Sentencing the 36-year-old Australian native, Judge Anne Molyneux said Oldfield had acted dangerously and disproportionately.
"You did nothing to address inequality by giving yourself the right to spoil the enjoyment of others," she said.
"In doing so, you acted without regard for equality and contrary to the meaning of it.
"You made your decision to sabotage the race based on the membership or perceived membership of its participants of a group to which you took exception."
A statement from four-time Olympic rowing gold medallist Matthew Pinsent, who was in a launch near the incident, was read out in court.
"He could have been killed if he had been struck by an oar or the rigging, which is metal," the statement said.
Oldfield, who admitted swimming in front of the boats, said he acted after reading of the British government's spending cuts. "It was a symbolic gesture to these kind of issues," he said.
The 158th race was eventually restarted with Cambridge going on to win after the Oxford crew broke an oar.