Being a pilot Jeremy you ought to know a surface covered in water wreeks havoc with aerodynamics. My Hanglider is an all Mylar wing and if it's wet oh boy oh boy do you know about it!
Sometimes it can, but most often the effect is small, it depends on the aerofoil section used.
If the wing section is a laminar flow, or supercritical one, then water droplets can cause a significant reduction in L/D, due to the separation point moving forward and a greater area of the wing being subject to non-laminar flow. For bluff shapes, like most vehicles, there is either no negative effect or perhaps a slight reduction in drag. The latter is due to increased surface roughness from droplets actually reducing drag by producing a controlled thickness turbulent boundary layer that acts to "lubricate" the surface.
The other negative factor for flexible fabric aerofoils is that when they are wet they handle very differently than they do when dry. Although my old flexwing microlight still flies OK when wet, the handling, particularly in pitch, changes markedly. One factor is that the swept Rogallo wing planform means that the wing C of G shifts aft when it gets wet (there being more fabric behind the wing C of G than in front of it), which slows the HOT speed a bit.
Nowadays, most aircraft designers try to avoid aerofoil sections with excessive sensitivity to rain droplet (or dead fly) surface irregularities. The last big design error I know of was one of the early Rutan canard designs. To reduce drag he used a laminar flow section on the canard which caused massive loss of pitch control when it rained. The design was quickly changed when the problem became apparent (being based in the Mojave I doubt that Rutan ever flew the thing in the rain!) and subsequent versions have all used a non-critical aerofoil section that is relatively unnaffected by rain droplets.
My own designs use sections that actually work slightly better when a little bit "rough", as do most other modern designs of powered aircraft. As far as I know, it's really only hotship gliders that still stick with laminar flow sections that need to be kept very clean and shiny to work effectively.