Currently, render flags (such as 'Primary Visibility') are not supported with fluids. Neither mental ray nor Maya Software use multiple importance sampling, whilst Arnold does. 
There will be some differences in the rendered image from Arnold. You will have to increase the Arnold light intensity by 4 * Pi when rendering with MtoA. It is not a good idea to compare Arnold renders with the Maya software render. Arnold uses area lights, and the Maya software does not, however mental ray does. Changing the exposure on a light (using linear decay both on the Maya fluid, and on the Maya volume) to 3.65 (that's approximately equal to a 4*Pi multiplication) resolves this. This is required because the volume cannot scatter more light than the incoming light. 
First of all, because the volumes are 'physically based' in Arnold, we have to divide them by 4 * Pi (area of a unit sphere).
Even when matching Maya's fluid attributes, there are some differences. When you render Maya volumes, by default it is using the lights set up on the fluid, not the lighting setup in the scene (for Maya software, while mental ray renders fine). Things to bear in mind when rendering Maya fluids: There are some limitations when using them. Overall, it was worth it, as the volumes look believable and I got closer to the final result, saving a lot of time downstream during compositing and creative finishing.Volumetrics are currently compatible with Maya's internal fluids and Fume FX. Further examples of customer work using Volumes can be found here. Fortunately, the test rendering in the interactive Arnold RenderView is remarkably fast. This gave me the desired length, but increased render times. Increasing the Volume Ray Depth allowed me to add multiple scattering bounces within the volume. Overall, it was worth it, as the volumes look more believable and I could get closer to the final result saved a lot of time downstream during compositing and creative finishing. Volume Ray Depth allows you to configure settings that limit the ray recursion based on ray type and can help you create dramatic differences to a scene or image. 
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