One way to make the AI build boats (note you may need "Frozen Throne" install).
Step 1: In the World Editor click on Module, then click AI Editor.
Step 2: In the AI Editor change the race to custom.
Step 3: Click on Building, in the AI Editor then click on Add Priority, then go to Neutral, and add Transport Ship.
Step 4: Save the AI data.
Step 5: Go to Module, then click on Object Editor, then go to Neutral Passive, next go to Buildings click on Goblin Shipyard. New click on Techtree - Units Trained, and add the Transport Ship.
Step 6: New click on File and Export the Object data.
Step 7: Go to Module, then click on Import Manager, click on File next click on Import File, open the "AI data" you seve from the AI Editor. Next import the Object Editor data.
Step 8: Go to Scenario, then click Player Properties. Now change Player 2 to a computer.
Step 9: Make some shallow water, then make a Goblin Shipyard and change the owner of the Goblin Shipyard to Player 2.
Step 10: Go to Module, then click on Trigger Editor. Now add a new Action, next click on AI - Start Melee AI Script. Next change the Player to Player 2. Next click on the map.ai new click on Imported then click ok.
Step 11: New put the new "Action" at top of list in the Trigger Editor.
Step 12: Save your map and go test it (note in the custom game setup the race for computer should be set to Human).