I just went through this process as a buyer. If you are the buyer, you really won't have to do a thing other than write checks- of course you will have to pay the surveyor and you will also be expected to pay the round trip charges on the Travelift. If the seller is there, he or she will operate the boat during the sea trial. You are really just along for the ride.
The surveyor will inspect the engine and associated systems- hose clamps, filters, fluids, etc- but will most likely not "survey" the engine itself- that requires a diesel mechanic and the survey report should note that. Electronics will be powered up and checked for proper operation, plumbing gone through, hull and deck sounded with a phenolic hammer and checked with a moisture meter. The rig will be inspected, sail condition noted, etc etc.
After the survey is done, you should get a verbal rundown on what the boat's condition is. Everything will be catagorized- "ER" means essential repair, "RR" means recommended repair, and so on. A written report will follow which the lenders (if any) and the insurance underwriters will want a copy of should you decide to proceed with the purchase. It's an interesting process- good luck with it!