Providing safe potable water onboard vessels presents particular challenges and contamination can occur directly from source waters as well as during loading, storage and distribution. Between May and October 2005, 950 potable water samples were collected from 342 ships docking at ports. Comparison with Guidelines found 9% of samples contained coliforms, Escherichia coli or enterococci and 2.8% had faecal indicators (E. coli or enterococci). Action levels of aerobic colony count (ACC) bacteria were detected in 20% (22 degrees C) and 21.5% (37 degrees C) of samples. ACC results from one-off sampling are not informative as this does not enable port health authorities to monitor ACC trends. They should be removed as a routine criterion for remedial action and vessels should adopt the WHO Water Safety Plan approach, whilst continuing to monitor water quality with public health-based indicators (e.g. chlorine residual, coliforms, E. coli and enterococci). Logistic regression analyses identified practices associated with water quality. Practices protective against coliforms, E. coli or enterococci in potable supplies were: good hose hygiene, processing water onboard, maintaining free chlorine residual at >or=0.2 mg/L. This emphasizes the importance of good hygiene during potable water loading and maintaining adequate disinfection of supplies onboard.