A sea trial of the vessel is a very important part of the assessment process; however, it remains as optional. I strongly recommend that one is included in your instructions but understand it is not appropriate for everyone for a number of reasons.
At sea the engines can be placed under load for at least one hour and run at maximum rpm during this period. The Engine, drive train and instruments can be assessed in operation and some defects can only be highlighted in this way. The steering is assessed and all electronic equipment can be operated and tested. The sails can be deployed and seen in operation.
The main reason why yachts are bought is to perform the very functions I have described above so I believe it makes perfect sense to have them scrutinised under these circumstances.
If you are unsure or undecided I will be happy to discuss options with you during our consultation.