I can't say, as I may never go through Amazon to get MGM+.
I know what the OP means when watching MGM+, and certain other services on TV through cable you get the MGM logo or pay provider logo, I am guessing that's what they mean by watermark. But it just seems odd for these providers to do it while streaming, but you never can say. I never saw it for Peacock, Netflix or movies/shows made on Amazon, I never used any pay service on Amazon so it could be different per pay provider as well.