Some links to articles put out by DUI/BUI attorneys. It appears to me that open containers are not a problem, but the boat operator needs to/should be sober. It looks like .08 will get you a BUI (boating under the influence) charge. There are also BWI (boating while impaired) which might have a lower threshold. The county SOs, state parks, and USCG can all enforce BUI/BWI charges.

http://dui.lifetips.com/faq/120648/0/is-an-open-container-illegal-on-the-boat/index.html

http://dui.lifetips.com/faq/120649/0/how-does-the-coast-guard-get-involved-in-bui-cases/index.html

http://dui.lifetips.com/cat/64005/boating-under-the-influence/index.html

http://www.boatinglife.com/article/Product-News/When-the-Water-Turns-Dry

http://www.duiwashington.com/boating_dui/wa_boating_dui.html

http://www.seattle-duiattorney.com/dui/bui.php

The following link will take you to the RCW on boating laws.

http://apps.leg.wa.gov/RCW/default.aspx?cite=79A.60

Here is the RCW on operating a vessel while under the influence.

RCW 79A.60.040
Operation of vessel in a reckless manner — Operation of a vessel under the influence of intoxicating liquor — Penalty.
(1) It shall be unlawful for any person to operate a vessel in a reckless manner.

(2) It shall be a violation for a person to operate a vessel while under the influence of intoxicating liquor or any drug. A person is considered to be under the influence of intoxicating liquor or any drug if:

(a) The person has 0.08 grams or more of alcohol per two hundred ten liters of breath, as shown by analysis of the person's breath made under RCW 46.61.506; or

(b) The person has 0.08 percent or more by weight of alcohol in the person's blood, as shown by analysis of the person's blood made under RCW 46.61.506; or

(c) The person is under the influence of or affected by intoxicating liquor or any drug; or

(d) The person is under the combined influence of or affected by intoxicating liquor and any drug.

The fact that any person charged with a violation of this section is or has been entitled to use such drug under the laws of this state shall not constitute a defense against any charge of violating this section. A person cited under this subsection may upon request be given a breath test for breath alcohol or may request to have a blood sample taken for blood alcohol analysis. An arresting officer shall administer field sobriety tests when circumstances permit.

(3) A violation of this section is a misdemeanor, punishable as provided under RCW 9.92.030. In addition, the court may order the defendant to pay restitution for any damages or injuries resulting from the offense.
[1998 c 213 § 7; 1993 c 244 § 8. Prior: 1990 c 231 § 3; 1990 c 31 § 1; 1987 c 373 § 6; 1986 c 153 § 6; 1985 c 267 § 2. Formerly RCW 88.12.025, 88.12.100, and 88.02.095.]
Notes:
Effective date -- 1998 c 213: See note following RCW 46.20.308.
Intent -- 1993 c 244: See note following RCW 79A.60.010.
Effective date -- Severability -- 1990 c 231: See notes following RCW 79A.60.170.
Legislative finding, purpose -- Severability -- 1987 c 373: See notes following RCW 46.61.502.