Hey Hey,
A lot of misconceptions and confusion here.
I hope I can clear up some, graduating in naval architecture with a lot of fluid dynamics.
First off: We have lift and drag on a hydrofoil. Hydrofoil works in water, for calculations we often need the viscosity and the density. Density changes little with temperature, viscosity a lot.
Now for lift, we can (almost) neglect viscosity if we are not near stall speed or stall angle (for stall viscous effects are really important) or have issues with the interaction of our foil with the free surface (ventilation for example).
Hence Lift can be approximated quite accurately with potential theory CFD codes which neglects viscosity, if we are sure that we don´t operate near stall, or with very thick profiles for example.
So lift will change veery little with temperature as it´s not dependent on viscosity (almost) and density does indeed not change much.
For drag, theres also different kind of drag for example:
-viscous drag (friction drag-> viscosity really big factor as the name says)
-induced drag (something with eddies and vortexes around our foil mainly driven by wing angle, twist and AR)
-form or pressure drag (foil pushing away the water kind of, pressures acting on our foil)
-interference drag, parasitic, cavitation, ventilation and so on
That´s why in engineering we need more complex and expensive CFD codes (RANSE etc.) or experimental testing to estimate viscous forms of drag accordingly if we can´t rely on empirical formulaes for specific fields.
Back to drag:
Here you also have to look at how big the different kinds of drag are in comparison to overall drag depending on what craft you look at. On a ship friction drag and wave drag are the most important. If you go slow friction drag is important, while when going faster wave drag increases a lot and becomes the main driver.
Even if a ship has very very very little resitance compared to the mass that you´re moving you want to know the drag quite accurately to know how big a motor you have to install to go the speed that you want, and small changes or mismeasurements have a huge impact on the fuel you are burning if you run 24/7 if you estimate the drag not accurately.
That´s where you need tank testing and also why I brought it up because to estimate the frictional drag you depend a lot on the temperature and viscosity that you´re going in to make a good result.
I maybe went too far off topic and did not explain properly.
On kite hydrofoil I can´t tell you the proportions of all the drag forms accurately, but friction plays of course a role if you want to be the fastest at high speed, and even more importantly probably ventilation on the mast is a killer which are both viscous effects and therefore depend a lot on temperature.
And that´s why people sand their boards with different grid depending on the viscosity and temperature, to help the flow a bit to stay attached before ventilating or be low frictional drag. Apparently different surface roughnesses have evolved to handle different temperatures better in practice.
The only circumstance I can think of where lift could change a lot with temperature is when you have a very poor flow almost or temporarily detaching on a foil due to a wrong surface roughness or bad finish and it seems to like a different viscosity better to stay attached and you realise a difference. That is though not a condition a racer would be happy to operate in