Water finds its own level, and a hose can siphon it uphill once it's full. Any hose that joins the sea below the waterline, and runs to something inside the boat that sits low, can flood the boat that way. Bilge pump discharges and heads systems are the usual places.
A vented loop is the fix. The hose is led up in a loop well above the waterline, and a small valve at the top lets air in, so the siphon breaks. Practical Sailor notes that under ABYC H-22, a bilge pump discharge that can't be placed high enough above the waterline needs a vented loop and a properly installed seacock.
What a surveyor checks
Each loop's height above the waterline, with the boat heeled or loaded in mind. The vent at the top, which clogs with salt or waste and then does nothing. Hoses that should have a loop and don't, especially bilge pump discharges through the hull near the waterline.
In the report
"Bilge pump discharge exits the hull about 100 mm above the waterline with no vented loop and no seacock. Fit a vented loop and seacock, or relocate the outlet higher." See ABYC H-22.