Advance
Dry Dock
It is a port building that provides the necessary infrastructure to construct and repair boats in isolated dry basins that can be flooded at will. This ease of access to the hull of the ship improves in great measure the efficiency and capacity of the building process.
Advance tree
The game files don’t say where this goes: it has no requires, so the game places it in the 
Discovery tree itself.
Definitionin_game/common/advances/1_building_unlocks.txt:775
780ai_weight = {
781if = {
782limit = {
783has_ports = yes
}
785add = {
786value = 100
787multiply = num_of_ports
788divide = num_locations
}
}
}
793ai_preference_tags = {
exploration
}