Paison: If an enemy landed on an island. Is there a way to attack them?
Strictly speaking: no. This is also (part of) the reason why seafaring is only available in a handful of maps (mostly campaign maps) and has been completely excluded from the game with the Mission CD. Once a "landing spot" (i.e. where your expedition ship builds the new harbour) has been occupied, you can no longer reach it from the sea - so if an enemy occupies an entire island, they are safe there. In order to attack, you need a sufficiently occupied military building nearby
on the same landmass. The same limitation applies to the anniversary edition of the game - even though seafaring plays a much larger part there, the developers never bothered to fix this. (It was finally fixed in the kind-of-sequel "Aufbruch der Kulturen" though.)
@Loverman
Getting a geologist from one harbour to another should definitely work although the entire ship system has been known to be buggy and frustrating at times. What you could try if the problem persists is this: navigate a geologist to the sending harbour (i.e. a harbour on the main island your geologist can reach on foot). You can do this via storehouse settings (send geologist out of storehouse where he is currently located and block storage of geologists in all storehouses except this one harbour). Retry ordering the geologist to the island once he is waiting in the sending harbour. (Obviously, always make sure that you do have free geologists somewhere. Calls do not queue so if you call a geologist while all your geologists are somewhere in the mountains, nothing will happen.)
