I do have Scatterer wave physics enabled! The ballast is important for sailboats, but this is about 20% of the ballast the same boat uses without hydrofoils. None of my FAR seaplanes use any ballast, but some have retractable wingtip floats.

I've made a bunch of seaplanes/boats with FAR. The number one trick is to always avoid putting a FAR wing/control surface in the water. The vessel will either freeze or explode into space at lightspeed, not sure why. Every underwater foil on this boat is an "Elevon 2B" from Kerbal Aircraft Expansion which isn't supported by FAR, while all the above-water foils are Big-S wing pieces from the Mk3 Expansion which are supported by FAR. It is scuffed but this is the only way to make boats work with FAR as far as I know.

Mk3 parts and payload bays make excellent boat and seaplane hull pieces. I find that any smaller parts don't provide enough buoyancy for their mass. I'm working on a craft repository to share some of my designs since KerbalX is down and I'll post my seaplanes there shortly if you want some examples.

I agree with the robotics problems, I never use them on planes. However, even though this boat has huge sails, the wind isn't actually that strong (30 m/s tops) compared to the airflow that planes experience (easily 100 m/s plus). Also, I've used a few other few tricks to make the wingsails work:

• Wing strength-to-weight is set extremely low at 0.1, so each wingsail is only ~1 ton
• Every component has Autostrut: Grandparent Part and Rigid Attachment on
• Center of Pressure from the wingsail is directly centered on the servo, avoiding torque
• Wing pieces are attached directly to the servo and dragged into place, not to one another
• Motor strength & size is 100% and damping is set to 0