Law enforcement patrolling the waters have significant authority, well beyond their authority with automobiles, to stop, detain, board and inspect any vessel, even without reasonable suspicion or probable cause. This reduced expectation of privacy for vessels is due to their mobility and the government's responsibility to determine compliance with administrative regulations and unique safety issues not applicable to automobiles. Therefore, there will often be no need by authorities to show evidence of bad "driving" or poor operation, and issues based on traditional Fourth Amendment concerns based on the stop, or a subsequent search may be non-existent.
Of course, at trial, the prosecution must still prove who was in physical control of the vessel, and do so beyond a reasonable doubt. This can be difficult in some situations, because vessels are often contacted as they float in the water and while the operator is not behind the helm, but in another area of the boat. In such cases, the state must get admissions, statements or some other evidence to prove that the defendant was in control of the vessel.
Once an officer determines who was in control of the vessel, and has grounds for suspecting impairment, the "personal contact" phase of any officer's NHTSA training would apply. However, factors found in boating on open water, are not necessarily issues in an automobile stop. For instance, when an officer observes and articulates "blood-shot, watery eyes" as evidence of impairment, one has to wonder how much weight that indicia should have when most boating is done on hot, bright, sunny days without any shade or protection from constant sun-glare off the water. Or, when an officer observes "balance" issues, the rocking of a boat may obviously be a factor. Even if the suspect is contacted on dry land, it can take time to overcome what many experienced mariners refer to as "sea-legs," and to readjust to a steady surface.
If the officer feels (rightly or wrongly) that there is evidence of impairment, then they will ask a boater to submit to field sobriety testing. This is often an area of great debate and argument when the standard field sobriety tests as outlined in the NHTSA manual are also used in a BUI investigation.
The first field sobriety test, which would be administered under the standardized "three-test" battery of the NHTSA guidelines is the Horizontal Gaze Nystagmus or HGN. Typically, this is a very effective test if the officer administers and interprets it in the standardized fashion as required by NHTSA. However, many officers do not administer it as required, and additionally, the rolling motion of waves can have an adverse effect on the vestibular system. A disruption to the vestibular system often invokes imbalance, vertigo, and visual motor disturbances.
Under the NHTSA guidelines and the three-test battery, the next test to be administered after the HGN would typically be the "Nine-Step Walk and Turn," (WAT), followed by the "One-Leg Stand" (OLS). For obvious reasons, many patrol officers will not administer these balance tests on a boat – further undermining any scientific legitimacy the NHTSA three-test battery may have. Instead, they will often substitute non-validated sobriety tests such as, reciting the alphabet, counting backwards, the finger-count test, or the hand-slap test. None of these "tests" have any scientific validity, or peer review credentials, and can often be attacked effectively in front of a jury.