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!