Thoughts? So needs a high degree of water resistance. The parts will ultimately be painted, and I'm assuming that will provide a level of UV resistance; is that a correct assumption to begin with? I also need to consider surface finish as it is an appearance model too. So a high degree of off-the-machine-surface finish OR the ability to be finished to a high degree. I'm a static model builder, so the need for functionality is a new element to me. I need to lean on someone's prior experience; what have you found is best? This is potentially going to be an expensive model, so I need tried and true techniques and I don't have any room for experimentation.
Usually for scale models, Fine Detail plastic is a go-to. How large is your model? For an RC submarine, I would assume it to be in the 1/200 range. If you want a larger one-piece hulI, you may have to use Versatile Plastic. Check the bounding boxes in the Materials Guide for each material. Polished Fine Detail may be smoother, but you may lose some of the crispness to surface details too. I think you are correct in saying that painting it will protect from UV damage. I think the overall size is going to dictate how the print process looks. Define functionality. Control surfaces like the rudder and dive planes would probably be fine having their bearing surfaces just be part of the print, but it may be worth considering having brass bearing for the propeller shafts and such. I'm not very much of a shipbuilder, but I'm assuming there are aftermarket parts for things like that. Nylon is generally low-friction, but it may not be worth the trouble and cost for smaller parts.
Functionality as a contrast; in static model building it just has to look pretty with a coat of paint. Here the customer wanted something that could be submerged, exposed to the elements (water, UV, heat). Anyhow, went with the Nylon just for the cost savings as this was on a 1/72 scale model. My part was just going to ride piggy back.