These waters are waist level
The hallway's flooded
Lost your scuba gear

Shark LL paints a hopeless picture indeed.  At some point you have lost your scuba gear, most likely because you are blind, crippled and crazy.  Ironically this is a very similar situation to what happens to LL Cool J’s character “Preacher” in the movie Deep Blue Sea.

The killer's cold-blooded
His name's LL
You don't really want it

He reminds you that you are being hunted by the Shark LL Cool J, now cold-blooded both figuratively and literally.  He knows you don’t want to die and he doesn’t care.

I ate your ancestors
The ocean is haunted

The timeline gets murky here, everything up until this point has lead us to believe that this whole man-shark hybrid LL Cool J is a rather recent thing, then how has he had the opportunity to eat your ancestors?  One can only assume that either this is just another boast or that becoming a man-shark has given him powers over time or at least access to a time machine.

I'm closing in cause I'm supposed to win
How the cold steel feel when it froze your chin

Man-shark LL Cool J believes in predestination.  Not only is he the stronger of the two he’s meant to defeat you.  Is this true or is he perhaps trying to psych you out again like when he suggested you not even bother to defend yourself with a weapon.

Should of stayed on dry land
Stroke while you can
Cause now you under pressure in the land of the damned

Assuming you have regrets, Shark LL lists them here.  You should have tried harder or never even left the safe bosom of land at all.  There is a clever play on words here where “under pressure” refers to both the greater pressure that is felt deep beneath the sea and also the extreme pressure experienced when you are about to be devoured by a shark man.

Abandoned pirate ships
Eels and sod scum
Fish that glow in the dark
The Titanic's hub
Underwater storms

Shark LL takes us on a tour of his new surroundings at the bottom of the sea.  Deep and dark is this blue world and also possibly in the North Atlantic.

You're blood is so warm
You're life vest is off
And that turns me on

This playful stanza lets the listener know that while LL Cool J is now half shark he is still all man and will most likely have his way with you before eating you and your family.

Killer for centuries
The Gotti of the deep

Again we wonder how newly created Shark LL has been able to stalk prey for centuries, giving credence to the theory that he now can either control time or has some way of altering history through time travel.  Then he refers to himself as the Gotti of the deep, which one can assume means he’s become some sort of undersea media darling who keeps the D.A. scratching his head. 

In the next millennium
I'm still gonna creep

Another shocking revelation, becoming a man-shark hybrid has made him possibly immortal, possibly with a greatly extended life span but most likely, taking into consideration previous stanzas, Shark LL now has control over time somehow.

Sand under my belly, ocean over my head
Through the light in the shadows
You become the living dead