Noticed it on a couple of games, sometimes during a war, it seems that the AI sets a goal to capture/attack some islands but seem unable to realize that it will never be able to achieve its goal since all tiles are occupied and there is no city to assault. 

The result is that, instead of prioritizing accessible targets it tends to keep a large army in areas that are eventually useless. 

What it could do instead: 

well it depends on a few things: 

1. the strength / number of the armies blocking the island,
2. the strategic value of the territory (luxury or strategic resources + distance to the AI empire's cities or Player's cities)

Depending on those it could either: 

1. abandon the area and send all its troops where there will be actual fighting (maybe just wait a few turns to see if the units blocking the land actually move),
2. leave a few naval units (barely stronger than the armies occupying the islands - using their embarked strength for calculation?) and send the rest of its troop where there will be actual fighting.

Another solution to the larger issue of being able to completely protect group of islands could be to give ranged ground units the ability to trigger fights from a distance? Like for example, if the AI could place a couple of gunners on the top two tiles then, those gunners could trigger a battle with my units on the islands below. I'm guessing it's a complicated suggestion that might create issues for regular ground battles though.

below: a save at the very beginning of this war (iirc the AI had already started to mass troops near those island right at the start) and a save just one turn before the above screenshot.

French Turn 157 war.ctr

French Turn 182.ctr