If a body of water is bordered entirely by land with no cities on it and another tribe is inside that water, there is the potential that you cannot get any units to their territory to conquer them.
The problem arises from the AI (or player) failing to take border growth so you are unable to create ports or move units onto the water. This is the 2nd game I've experienced this; when taking over AI-owned cities that are size 5+, you can't go back and choose "border growth" to fix the problem.
A fix could include allowing the player to trade "walls" for "border growth" on captured cities, with some sort of cost (stars, loss of creating a unit that turn, etc.). Or, just make border growth an option each time after size 5 along with parks and super units.
The following screenshot demonstrates this. I cannot get onto the water because the two previously AI-owned cities (Halim and Romopipi) cannot be grown, and there is no way to place a port. The game is unwinnable at this point -- all I can do is kill the AI units as they come into range.