The premise

A mobile marine mechanic's first visit is diagnostic. You can't carry the shop, but you can carry enough to diagnose 90% of calls in one trip — which is what makes the business model work. This is the field-tested kit list.

Diagnostics

• Multimeter with DVA/peak-reading adapter (ignition work on older two-strokes)
• Adjustable spark gap tester (a plug on the block is not a test)
• Compression tester with the common thread adapters
• Fuel pressure gauge kit with schrader and T-fittings for EFI rails
• Vacuum/pressure hand pump (lower unit seal testing, fuel primer checks)
• Infrared thermometer (overheat calls, riser comparison)
• Borescope (cheap USB type is fine — cylinder walls, tanks, inaccessible bilges)
• Diagnostic software/cables for the brands you service, plus a rugged tablet
• Clear fuel hose sections and spare primer bulb (the world's simplest fuel-delivery test tools)
• Battery load tester

Almost-always-used consumables

• Common fuses, tinned wire, adhesive-lined heat shrink, quality crimper
• Water separator filters (the two or three most common), inline filters
• Spark plugs for your area's common engines
• Gear lube + pump, engine oil, TC-W3, greases, penetrating oil
• Impeller kits for the top sellers in your territory (know your local fleet)
• Cotter pins, prop nuts kit, stainless hardware assortment, hose clamps

The stuff people forget

• Muffs and a water source plan (ask the customer before the visit)
• Kill-switch lanyard spares (multiple brands — dead lanyard, dead diagnosis)
• Prop wrench, blocks of wood, drive/gearcase support
• Headlamp, mirror on a stick, magnet on a stick
• Absorbent pads and a small spill kit (professionalism and the law at the dock)
• PFD for sea trials, sunscreen, drinking water — heat exhaustion ends workdays
• Waterproof notebook or phone workflow for photos of every finding before and after

Process notes

• Photograph serial tags on arrival; half of parts ordering pain is a bad model ID.
• Ask the three questions before loading the truck: What does it do? When did it start? What changed right before? The answers pick which kit modules ride along.

Common mistakes

• Carrying repair parts for everything and diagnostic tools for nothing — diagnosis is the product on visit one
• No spill containment when the fuel system opens
• Skipping the sea-trial PFD because the customer isn't wearing one either