A person interested in looting the Towers needs to go the Wharves office to register they interest. They'll then get a list of licensed boat owners that can take a willing person to the Towers, that are surrounded by the barges. The going rate for an expedition is five gold per person transported.
The trip is about 24 hours each direction and to get to the actual towers, one needs to have their vessel inspected by three Tower Guards. They make a list of all items on the boat.
Once you get through, you can be at the Towers as long as you want. When leaving, you'll be inspected again, and everything that isn't on their original list is considered to be recovered from the towers. The Tower-scavengers get a new list containing a short description of these recoveries, which is closed with a seal. Breaking a seal is a crime of property, and will result in fines.
You then take the treasure and this list to the Excise House, where everything is checked against the list and appraised by the clerks and you'll be awarded half of this value in cash. Items on the list that aren't present in the items are considered to have been stolen which, again, is a crime of property.
Upper-echelon clerks (like Tremance) arrange the items into chests, make a single list of the goods in a given chest and its worth, and it is then transported by the city guard to Exotic Treasures to be sold (so the Council gets the profit they "deserve").
Covis is a boat owner by trade so all he's supposed to do is transport people and loot back and forth and pay his license and mooring dues.