Ethan Daniel Baker, 21, pleaded not guilty to four counts of assault.

This week magistrate Simon Brown dismissed three charges but found Baker had attacked a fourth man at the Longford boat ramp on January 19 last year.

"He clearly stood out. He was cleary drunk and clearly aggressive," Mr Brown said.

"Mr Baker's threats after (the victim) emerged from the river showed a particularly high level of aggression".

The court heard Baker was involved in holding the man underwater and "kicked and punched him and threatened him while he crawled out of the river as well as while he was still under the water".

Baker was among a group of 10 to 15 young men who had been at the Country Club Hotel at Longford before walking down to the boat ramp.

A number of others were charged with assault, including a man who was a youth at the time and so cannot be identified, but Baker was the only one found guilty.

Mr Brown found three other men had been assaulted by the group but on the evidence he was unable to identify by whom.

The court heard that as the group left the boat ramp, somebody yelled "don't mess with the Baker boys".

Baker will be sentenced on June 10.