Rudder and daggerboard section selection is not only important in theory. Some years ago we replaced the original rudders on a Tornado. We changed from the classic thin profile to a thicker foil with higher cavitation resistance but still the same drag level which made a huge noticabale difference to boat handling and speed. It removed all weatherhelm we had a high speed, which was significant and drag from the partially cavitating rudders.

Don't select laminar sections as they are inferior when operated in turbulent conditions and surface water is always turbulent if there are waves. That's why most people are happy with NACA 4 digits sections.

Concave sections are difficult to build, hence better stay with simple convex sectuion. Again a good reason for NACA four digits.

Other than stated here in this forum, skin smoothness or quality is very important, 1 mm roughness would have an catastrophic impact, and 0.1 would be still significant. A foil should be as smooth as possible and smoothness is more important than the section.

Cheers,

Klaus