Rob
Ansys Employee

For the graphics, in 2d don't pick any surfaces. You'll get a continuous contour as the solver will magically figure it out. 

I can't tell if you have supports under the bridge, remember in 2d there's no way for the water to get around these: that could cause some interesting results. If they're buoyant cells that may be different. 

Residuals & reports are mostly as expected. You may need to resolve the mesh near the free surface, but I can't see from the image. Generally in multiphase there is something in the flow (reflected waves, ripples etc) that are transient. The question you need to consider is whether these waves etc are altering the data you're interested in. Ie are you wanting to see the flow field and understand the range of drag, or see the speed/rate of any transient effects?