On page 23 of the rule book, during Phase 6: Collect Income, it says you calculate income, bonus income (national objective income, war bonds income if you have the technology) and lost income to convoy disruptions. So it is then that you determine damage. (This also means that each nation can take damage from enemy ships from the same sea zone, if applicable, as it is calculated during the collect income phase.)
Page 24 explains how convoy disruptions are conducted.
First, make sure that the sea zone in question has a “convoy” image.
Second, make sure the sea zone is next to an enemy territory (that enemy must have it’s capitol in friendly hands, this is stipulated on Page 23)
Third you must have at least one surface warship (not an Aircraft Carrier), planes or submarines in the sea zone being disrupted.
Lastly, throw the dice.
Submarines, Fighters and Tactical Bombers get 2 dice each.
Battleships, Cruisers and Destroyers get one die each.
Take all the dice, throw them, tabulate all the numbers less than or equal to “3” on the dice. (so if you threw 1, 4, 3, 6, 6, 5 then only the 1 and 3 would count, and total 4 damage.) Deduct this amount from the person’s collect income phase - or the maximum amount of IPC the territory(ies) adjacent to the sea zone are worth.
Like SBR, you can never lose more to convoy raids than you can to a complex.