There is a company called IGUS that specializes in plastic bushings. They recommend a smooth but not too smooth shaft. Somewhere in their catalog there is better information but off the top of my head (and it's been several years since I've designed shafts for plastic bearings) I think it is around 32 micro inch rms. There is a little more information in the image below and at this link: http://igus.bdol.com/igcat/ig_cat1.asp?p=ig_1_8.gif

IGUS has a line they call H370 for underwater applications. http://www.igus.com/igubal/igall2.asp?p=H370 They say their plastics are proprietary so they don't list material types unfortunatly.