I'm looking at a boat with dual 2005ish 350 MAG 5.7L, long block rebuild 3-4 years ago (supposedly).

Port
1. 112
2. 118
3. 118
4. 115
5. 110
6. 112
7. 115
8. 118

Starboard
1. 110
2. 115
3. 120
4. 120
5. 108
6. 122
7. 110
8. 118

I'm no expert, while the uniformity looks reasonable - the numbers are low? Should I be concerned?

Exactly my reading too...
are you sure your gauge is accurate?
if so, engines are getting more towards their 2nd part of life vs the beginning...

could run like that for years!

Was that on a cold engine and was the throttle pinned open?

The test really should be done on a warm engine, throttle/choke open.

If they were rebuilt, I would expect more even cold.

I vote for gauge being off a bit. Perhaps the throttle not WFO or low cranking speed? I would not think if it was that worn that the numbers would not be that even. I would think it should be 150ish or so.

Throttle was my thought too.. I've seen so many videos of people compression testing with a fully closed throttle. Gauge is also a good possibility... Get a new gauge before digging into it...
Another thought... If you haven't, pull all of the plugs before testing, that will allow the motor to spin as easily as possible.

Consistency is decent, 10% is optimum. You are at 12%. Those numbers do seem low though, my 97 350 magnum (carbed) was 155+- (warm)

How strong were the starters/batteries turning them over and we're they rebuilt with the original cams?