I find the seatrial to be more beneficial for things like operating the engine at cruise speed, which not only tells you something about the motor, but also the shaft, transmission and prop. That can't be done at the dock or in the slings. It is also the only way to really test some systems, like the autohelm, if you have one. You might want to test the VHF from the water. It is worth going out.
As far as sailing characteristics go, I've not really felt any surveyor (good or bad) was extremely helpful in this regard. A glaring problem should be noticed, such as a rigging problem or worn sails and that has value if you are buying your first boat. They just can't be familiar with every hull and sail plan out there to know every subtlety and you are naturally only going to sea trial in one wind condition.
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In the harsh marine environment, something is always in need of repair. Margaritas fix everything.