Hi!
The Arnold Sequence Render does not add a watermark, but it basically just renders everything in the renderview and then saves the files, which takes up a lot of memory, so you can't really use your pc for anything else while this is happening and it needs the scene that is rendering to be actively running. Also since it basically just renders out the renderview it is fairly limited, no transparency, AOV's etc... So this is an option for small and simple projects, but its not optimal...
As far as I know there isn't a good free render plugin for maya right now, sadly...
The cheapest maya render plugins are probably maxwell and IRay
https://www.irayplugins.com/iray-for-maya
https://maxwellrender.com/
Since both those renderers have one time purchase licenses instead of monthly/annual payment plans.
Something else you can do, is export your Scene as alembic cache and then import and render it in another 3D Software/Game Engine that has a free renderer ( for example: Unreal,Unity, Blender) and render there.
As far as I know, Maya has a Plugin that allows you to work with Unreal really fast called "Unreal Live Link"(https://apps.autodesk.com/MAYA/en/Detail/Index?id=3726213941804942083&appLang=en&os=Win64&_ga=2.2337... I personally never used this, so I'm not sure how well it works. But it is probably the way to go if you want to render in another software...
This method is free, however it will probably mean you have to do all the lighting/texturing steps in the other program, so this will take a lot of additional time to learn how to use the program and render with it, also it is a bit of a wonky method to do everything in maya, then go to another software and potentially do some of the steps again (depending on what program you choose).
So if you do alot of rendering, I would consider going for one of the cheaper maya render plugins, since the time you save by not having to export will eventually make up the cost of the plugin...
If you don't render frequently, using another software for rendering might be a good viable option, I know quite a few animators that render their reels in another software, so I'm sure there are also good tutorials on this out there.
I hope it helps!