Stringers run fore and aft along the inside of the hull bottom, like the joists under a floor. With the bulkheads and frames they make the grid that stops the hull flexing, and on most powerboats the engines sit on them.
Many are a wood or foam core wrapped in fibreglass. The glass keeps water out until a screw or a drain hole lets it in, and then it can't get out. David Pascoe pointed to unsealed fastenings and boxed stringers with no drainage. Wet wood rots inside a shell that still looks fine.
What a surveyor checks
Each stringer along its length for brown weeping and places where it's lifting from the hull. Sounding it, and the sole above for soft spots. Where there's doubt, Discover Boating's test: a small hole drilled into the wood. Wet or rotten shavings mean rot. The engine mounts get special attention, because rot under a mount moves the engine.
In the report
"Port engine stringer, test hole aft of the forward mount gave wet, dark shavings. Stringer firm to sounding." Give the position, how you tested, what came out and whether it's still stiff. See stringer rot.