Both the tech-map and the in-game description say that only dockyard is required for shipbuilders' guild. The in-game description further states that compass is required to build it. Only in the txt file can you find out that cannon foundry is needed as well. You need it make all the ships onwards though, except for the wargalley.

The weird thing is that Italians have it before gunpowder is discovered if you start a high period game as them. You can build wargalleys before you are normally supposed to in that period.