I actually get quite paranoid about the number of "neutral" navies swarming around my shores. For the most part they are there to pick up diplomats and ferry them off to some other place in order to attempt more useless briberies. However, I have seen quite a few seaborn invasions. I will echo what someone said earlier though, I have never seem the AI take Rhodes, Corsica, or Sardinia and this is after playing 100+ turns in 5 campaigns.