What the inspection covers
A pre-purchase inspection is a mechanical and electrical assessment of a boat you do not own yet. The engine comes first: cold start behaviour, idle quality, oil condition and level, cooling flow, belt and hose condition, fuel lines and primer bulb, battery state and charging output under load. Then the drive: gear lube condition, prop and prop shaft, trim and tilt operation and whether it holds position once set. Nothing is taken on trust, and nothing is judged from the listing photographs.
Most of the value is in the things a seller has no reason to mention. A trim ram that weeps. A steering cylinder with a torn boot. Corroded ring terminals behind a battery switch. A bilge pump wired through a float switch that no longer floats. None of these stop a boat leaving the dock on a ten-minute demonstration ride, and all of them are money once the boat is yours. Everything found is written down, so it is not a memory of a busy afternoon.
Where a mechanical inspection stops and a surveyor starts
This is not a marine survey. A survey by an accredited marine surveyor is a different document with a different purpose: it establishes value and condition for an insurer or a lender, and it usually involves an out-of-water examination with moisture readings across the hull and a structural assessment of stringers, bulkheads and coring. Insurers and banks frequently require one on older or higher-value boats. If the boat you are buying needs financing, needs a policy written on it, or is large and old enough that structure is the real question, get a survey.
A mechanical inspection answers a narrower question extremely well: does the powerplant and everything attached to it work, and what will it take to put right what does not. On a twenty-foot bowrider with a known engine, that is usually the whole question. The two are not in competition. Buyers of bigger boats often arrange both, and the mechanical findings tell the surveyor where to look harder.
Compression, gear lube and the story fluids tell
Compression is the cheapest window into an engine's internal condition. What matters is less the absolute number than the spread across cylinders: readings inside a tight band suggest even wear, while one cylinder well below its neighbours points at a ring, a valve or a scored bore, and is a reason to stop and investigate rather than to negotiate. On a four-stroke, oil condition and level are read alongside it. Milky oil, a level that is too high, or a fuel smell all change the conversation.
Gear lube is the other tell. Drained into a clear container, healthy lube is translucent; water turns it opaque and grey, like coffee with milk, and metal glitter in it means gears or bearings. Water in a gearcase almost always means a failed prop-shaft or driveshaft seal, and it says something about the seller as well as the boat. A lower unit run through a whole season with water in it is not a lower unit that has been looked after.
Electrical, hull and transom
The electrical walk-through follows the current. Batteries load-tested rather than read at rest, terminals off and inspected, the main switch and fuse block checked for green corrosion and heat marks, charging output measured at the battery with the engine running. Then every circuit is operated: navigation lights, blower, horn, pumps, trim, gauges, electronics. Wiring added over the years by previous owners is where problems live, and it is visible at a glance once you know what a factory loom looks like.
Above the waterline the transom and hull get everything that can be checked without a lift: transom flex with the drive trimmed, the seal around the transom assembly or the outboard bracket bolts, stress cracks at the corners, water staining in the bilge and under deck hatches, soft spots underfoot. What needs the boat out of the water is the running surface, the keel and strakes, and any fitting below the boot stripe. If those matter to the deal, arrange the haul before money moves.
The water test, and what you are handed at the end
A boat that runs on a flushing attachment in a driveway has told you almost nothing. Load is what exposes problems: an engine that overheats only when worked, a drive that will not hold trim, a fuel delivery fault that shows up at wide-open throttle, a gearcase that whines under power, a hull that porpoises because of a bent prop. A water test puts the engine under real load through the whole range, including a hole shot and a sustained cruise.
That matters here because of the calendar. Most private sales around Lake Winnipesaukee happen in a short window either side of ice-out, on a boat that last ran in October and has sat on a trailer or under shrink wrap ever since. Ice-out in 2026 was declared on 13 April, and across the record the average has moved earlier, from roughly 23 April towards 14 April. Before that date there is nowhere to test, so a boat sold in March is being sold on an engine nobody has heard run under load.
If a water test is genuinely impossible, that is not automatically a reason to walk away, but it is a different risk and it should be recognised and documented as one. What you receive is a written record: every item checked with its condition, photographs of anything wrong, and a plain separation of ordinary wear, repairs to plan for, and findings serious enough to walk away from.