To prove the offense of Boating under the Influence, the prosecution must prove, beyond a reasonable doubt, that the person was operating a vessel while under the influence. “Under the influence” means having a breath or blood alcohol test result of 0.08 or higher, or being “affected by” intoxicating liquor, any drug or a combination of the two, to an “appreciable degree.”
I have not specifically seen this issue, but would imagine that an "anchored" defense would be similar to a defense of being "Safely off the roadway". The key is that the prosecution will have to show you were actually "operating the vessel". Have a safe and fun time at the races!
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I generally agree with Ms. Azure's answer, but wanted to write separately to add some additional information.
RCW 79A.60.010(16) defines "operate" as "to steer, direct, or otherwise have physical control of a vessel that is underway." RCW 79A.60.010(28) defines "underway" as follows: "'Underway' means that a vessel is not at anchor, or made fast to the shore, or aground."
So, by definition at least, a boat that is "at anchor" is not "underway" for purposes of Washington boating under the influence ("BUI") law.
By contrast, federal regulations regarding the subject, while not controlling of Washington BUI law, indicate that "an individual is considered to be operating a vessel when: (a) The individual has an essential role in the operation of a recreational vessel underway, including but not limited to navigation of the vessel or control of the vessel's propulsion system. [AND/OR] (b) The individual is a crewmember (including an officer), pilot, or watchstander not a regular member of the crew, of a vessel other than a recreational vessel." See 33 CFR § 95.015.
I hope this answer helps you create a fun and (most importantly) safe atmosphere in which you and everyone around you can enjoy this weekend's festivities. Please don't drink and drive or operate! Happy boating!
Each case is different, if I were on the boat with you and you were drinking, I would suggest you not get behind the wheel if it is anchored or seat yourself in the captain's chair as you would inevitably draw attention from the numerous patrols. That being said, it would be interesting to me if you were cited while anchored, but you could be cited by these patrols just to give you some trouble and make you go through the process.