I’m going to install a through-hull water pickup to attempt to solve an 8.1 cooling problem (really long story). My question is whether or not I can simply cap off the water intake pipe from the Volvo Penta DP-SM outdrive, or does the outdrive depend on that water intake for cooling?

Drives will benefit from extra cooling in the housing.

Is there an impellor in the leg, or does it use the raw water pump on the engine to draw it up?

With dedicated raw water draw, guys often plumb the drive raw water to a through hull and use it as a drive shower with a through-hull fitting.

The outdrive is not cooled by the water intake. Why don't you try and fix the cooling problem. That is a pretty simple setup. Pump mounted on the front of the engine, easy access.

No impeller in outdrive.

There’s a 7 page long thread on this particular engine with lots of photos and detail titled “(Another) 496 overheating issue”.

not sure how to add a link to another post

Click the chain link looking thing when doing new post. It’s in the upper right.
Anyway
Here is the link to your other thread:
(Another) 496 Overheat Puzzle

after reading through you original thread with your over heating issue it sound like you replaced everything except your circulating water pump on the front of the motor you do mention that you took it off and apart and every thing looked good. I also had a similar over heating issue and went through pretty much the same process i also took my circulating pump apart and found impeller in good shape and tight on the shaft but since i already had it apart i went ahead and replaced the circulating pump and low and behold it solved my problem i can only think that under load it must be enough to allow the impeller to slip on shaft you might try replace circulation pump first before jumping through any more hoops, i know what your going through chasing a over heating issue drives you crazy good luck.

later

I guess there’s a chance it’s slipping. I should have scribed a line on shaft and impeller as a reference. Sure don’t see any signs of slipping. What’s another couple hundred at this point. Here’s a before and after cleaning. I had a 19 second video but it was “missing a security token”.

I am sort of in same boat
I have taken everything apart,
1988 pump re-circulation that is this has Vanes and is solid
the new pump vanes are about 1/4 longer and coated with what looks like some hard coating
Boat is only freshwater
112 is not bad for a sierra pump

New pair arrived late yesterday..amazon🤣

1/4” longer vanes should make a big difference.

I’ll have a look at Sierra pumps, thanks.