Depending on what the cargo was, large parts of it would be ruined by the sea. Grindylows probably stole some of it.
Some things I would ask myself (and the players) before just handing out gold: Why did the bugbears not steal most of the useful cargo, like wine and food before sinking the ships? Why would all the ships in the harbor be full of cargo anyway? Usually the cargo would be offloaded pretty quickly to be sold.
To whom would they be selling the ships? And how? Are they going to sail all the ships themselves to Ghastenhall (or where ever)? Even if they manage to find buyers and deliver the ships they can get like 5000 gp per ship. Not that much for all the time and effort.
Repairing A Vehicle: The fastest and easiest way to repair a vehicle is with the mending and make whole spells, but more mundane methods can also be used. Craft (carpentry) can be used to repair most vehicles made of wood; because of their specialized construction, water vehicles require Craft (ships) to repair. Depending on the nature of the damage, such skills like Craft (cloth), Craft (alchemy), Knowledge (engineering), and even various professions can be used to repair vehicles, if the GM approves. In general, a day’s worth of work by a single person using the appropriate skill to repair a vehicle requires 10 gp of raw material and a DC 10 skill check, and repairs 10 points of damage on a success, or 5 hit points on a failure.
A galley has 1560 hp. If it is so badly damaged that it has been sunk it has been reduced to 0 hp and destroyed at -104 hp. Destroyed vehicles can not be repaired. So it takes one man 156 days and costs 1560 gp in materials to fix a sunken galley. There probably is some spare lumber in the ship yard. Or rather there was because it was partly used by the defenders or burnt for giggles by the goblins. Time to organize a bugbear lumberjack expedition.
In conclusion they should not bother because it is not very cost effective and they probably have better things to do. Like killing the king.