A sobriety test capable of being administered on boats was developed as an aid in establishing probable cause to request a breath sample of recreational boat operators suspected of being over the legal alcohol limit. In a laboratory phase some 12 candidate measures suitable for use in a boating environment were assessed for their relationship to blood alcohol levels through administration to 32 subjects tested at blood alcohol concentrations (BACs) of 0.00, 0.06 and 0.12%. Four measures showing significant relationships to BAC were then administered by marine police to a sample of 60 recreational boaters found to have BACs ranging from 0.0 to 0.20%. Three measures still showing significant relationships to alcohol were horizontal gaze Nystagmus given in a seated position, reciting the alphabet from A to Z, and clapping hands alternately with palm and back of hand, while counting. Correlation of the measures with BAC, and the ability to distinguish boaters over and under a 0.10% limit were similar to those found over the same BAC range with the standardized field sobriety test (SFST) for motor vehicle operators. Where there is not sufficient cause to request a boat operator to proceed to shore for testing, a standardized boating sobriety test (SBST) making use of the three measures appears to be a suitable replacement for the SFST.