Since Aboleths can have unlimited numbers of permanent slaves, and you're using him as a Big Bad, it seems important to thresh out exactly how big and bad this Big Bad is and under what circumstances you will fight him. If all he's got is a colony of ogres, and the players fight him in a little underground grotto where he's dragged himself up on land, then yeah, he could be a plausible fight for a bunch of 8th level PCs as long as they have some way to deal with his Charm. If, on the other hand, he's played up to potential as an actual Big Bad with tendrils everywhere and minions and millions of years of experience (Aboleth = inherited memories), who lurks in the water beneath and only occasionally visits his worshippers when they summon him from the depths, then he could be a real nightmare even for 14th level PCs, especially once he dominates a few PCs to add to his collection of giants/krakens/sea monsters. 8th level PCs could still potentially beat him if they ambush him but it would take brains.
I see Aboleths as something akin to Moriarty from the recent seasons of Sherlock. Asking what CR Moriarty would be missing the point. (CR 3? Watson could probably kill him in a fight.) If it ever comes down to simple combat between him and you, either you've already got him beat or else he's got the game rigged somehow so that killing him is actually a loss for you. Aboleths are quite capable in straight-up combat, but I'm saying that straight-up combat isn't how I would ever play them: the simple combat happens only after you defeat his army of scrags and are trying to prevent him from getting away to fight another day.
But yeah, in a cage match, on land, a party with a way to deal with Charm (Devotion paladin) could kill an Aboleth at 8th level. CR 10 means "at 10th level, in a cage match, this monster should be no more than a mild challenge (Medium), and a party could handle 4-6 of them per day."
