Field of Glory: Empires is a grand strategy game in which you will have to move in an intricate and living tapestry of nations and tribes, each one with their distinctive culture.
Set in Europe and in the Mediterranean Area during the Classical Age, experience what truly means to manage an Empire.
I got a bug where there are units in a lake tile which don't vanish....
I control all the area around it (have been for >10 turns) still the units are there...
There where a few different factions near the lake, don't know which one it is... I conquered all except one (Parthia) and they will be gone soon...
The AI can't lose units to being out of supply, unless they're in a besieged city (there's a checkbox in the options menu to turn this rule on for players as well). I see three possibilities.
1: It's a Parthian fleet and will disappear once you wipe out Parthia and they have no surviving allies.
2: It's from one of the factions you conquered and still in play because of the mechanic that allows factions that lose all their territory to keep playing with any surviving units if they still have a surviving ally. In this case you would still be at war with the offending faction.
3: A pirate fleet spawned there. Pirates don't care about supply, but I don't know if they're allowed the spawn in one-seazone lakes.
If it's pirates, the only way to get rid of them would be to embark land units into the seazone, causing a naval battle to occur. Naval transports tend to suffer heavy casualties when fighting against combat ships, but can overwhelm them with enough numbers - best to send a mass of cheap units.