Hi David,
As far as units the usual mistake can be air pressure not being in Pa or wind not being in m/s.
Is your wind sensor height correct?
If the met station is on the lake then it is normally best to turn on atmospheric stability.
What type of long wave radiation data do you have?
SURF_HEAT_TRANSF_COEFF and mean_albedo are the primary coefficients for controlling heat transfer but we never really change these if the data is good.
Do you have any inflows which could be effecting temperature?
Chris