98 Bonneville - Turns over slow & voltage drops

I'm working on a 98 Bonneville with a 3800 series II engine.
The car was parked & the next day it wouldn't start.
Battery & some of the cables are new, battery has been tested & is good.
When attempting to start, the voltage at the battery drops to 9 volts & the engine turns over really slowly, but it does turn over.

Is it possible that the starter isn't getting enough voltage?
Would that explain the significant voltage drop at the battery while turning over?

I'm going to remove the serpentine belt & see if it turns over a little easier, maybe a pulley froze up. What else would cause such significant resistance that the engine would have a hard time turning over? Before I got involved, I thought it might have been hydro-locked, but it does turn over, just really slow. The last time I had a car that hydro-locked, it wouldn't turn over at all.

Any suggestions would be appreciated.

A bad starter can cause this, battery also, or as you said water in the engine, see how it turns over on the crank bolt.

Turns out the voltage drop was normal at the starter.
Loosened the serpentine belt & found the cause.
The AC compressor pulley and/or clutch are frozen.
Car turned over normally with the belt removed.
AC didn't work anyways.
I'm going to pick up a used compressor for $25 at the local pull a part.