i don't know if this has anything to do with the AI not attacking the base (since the AI's race is Naga and it can actually traverse through water), but I read somewhere (i think it's in the Tutorials section like Spinnaker's link, too) that an AI will usually move to its location via a land path. if there is no land path (the location is in an island, or maybe bounded by cliffs) the AI will think it is unreachable, and that also an AI cannot use airships. try placing a Way Gate. i saw in several melee maps that AI actually use them.