Final Campaign Boss Battle

My campaign is coming to an end soon. It is a heavily homebrewed version of a krakens gamble. The final fight in a krakens gamble is with oosith, an aboleth. However, my bbeg was someone who murdered the current waterbaron and usurped the throne. He has now been kidnapping nobles around the city to keep the other houses quiet and in line. He has also allied himself with the Kraken Society. He conquered the neighboring town of Triboar to get the lords alliance's attention and plans to summon a kraken in the dessarin river once the Lord's alliance armies arrive to destroy them and put Yartar on the map as well as establish his own houses name. He wants fame and power. 

Anyway's my party is about to enter the sewers and I am struggling to balance the final fights. I want it to be epic and cinematic and very difficult. I have 4 level 8 players, and they have been doing everything they can to get allies to assist them in this fight. They will likely have a cr 6 mage ally, a rogue ally (i don't have a stat block for him) and one of the players through a whole previous campaign gained a dragon wrymling companion thought it is only a cr 2.

I have attatched a sketch of the final boss fight I am thinking of using. There will be a water rising mechanic, where the players will have to decide if they want to run straight for the bbeg or go to turn the water off so they don't drown. There will be 2 merrows, the aboleth, and at the back of the room the bbeg who is a kraken themed boss. I was thinking that the bbeg was using the life essences of these nobles he captured to fuel his ritual to summon the kraken.

Does anyone have any tips for me? or any criticism? I feel very nervous. I really want this to be memorable and don't want my players to be able to just roll through it all.

Thank you!

So, the biggest problem with designing epic boss encounters in D&D is... D&D has a high level of randomness, and there's also a lot of unknowns about how the players will approach an encounter. What this means is that an encounter you expect to be hard but beatable could turn into a TPK (the players fail to figure out a trick, or roll poorly, or the monsters roll well) or a cakewalk (the players come up with a idea you hadn't considered, or roll well, or the monsters roll poorly). There are means of mitigating this, but they all have their own issues, because part of what makes an encounter epic is the feeling that you're one mistake or bad roll away from disaster.

Quote from Pantagruel666 >>

There are means of mitigating this, but they all have their own issues, because part of what makes an encounter epic is the feeling that you're one mistake or bad roll away from disaster.

One bad roll away from disaster isn't exactly a bad thing. It is actually ok to have the party fail/die. If death weren't part of the deal then it makes the game less fun.

I'm not saying you should kill your PCs all the time, just have them know they are not immortal. Some GMs are afraid of having any of the PCs die, but really it's ok.

Quote from Wysperra >>

One bad roll away from disaster isn't exactly a bad thing.

 In a campaign, depending on the style of the game, sure. However, while the campaign where this occurred might be epic and cinematic, a one-sided beatdown (on either side) does not make an epic and cinematic encounter.

Makes sense what you are saying. I guess I will just have to roll with what I have and hope everything works out. Sometimes I am tempted to pull monster punches because I feel like I overtuned it, then the players end up rolling the monster and I wish I never pulled those punches. Maybe it's just an experience thing, as I am still a relatively new dm.

You mention the BBEG is using the captured nobles as fuel. What if those nobles are still alive? Like maybe they're sealed inside containers that are connected to the BBEG's power source. So now the players are faced with another choice. Do they all attack the BBEG directly? Or do some of them attempt to free those nobles, thus depriving the BBEG of the power needed to complete his ritual? 

I really like this idea. I definitely want to add more depth to the combat. Maybe I need a timer that starts once the final bbeg fight happens and if it goes up the kraken is summoned and the players lose