Here is a quote from the case regarding jet skis in the San Juans, and a few links to more info. about the PTD.

John Weden et al. vs. San Juan County
SUPREME COURT OF WASHINGTON
135 Wn.2d 678; 958 P.2d 273; 1998 Wash.
July 9, 1998, Filed

…

Since as early as 1821, the public trust doctrine has been applied throughout the
United States "as a flexible method for judicial protection of public interests in coastal lands and waters." Ralph W. Johnson et al., The Public Trust Doctrine and Coastal Zone Management in Washington State, 67 Wash. L. Rev. 521, 524 (1992). The doctrine protects "public ownership interests in certain uses of navigable waters and underlying lands, including [*27] navigation, commerce, fisheries, recreation, and environmental quality." Johnson, supra, at 524. The doctrine reserves a public property interest, the jus publicum, in tidelands and the waters flowing over them, despite the sale of these lands into private ownership. Johnson, supra, at 524. "The state can no more convey or give away this jus publicum interest than it can 'abdicate its police powers in the administration of government and the preservation of the peace.'" Caminiti v. Boyle, 107 Wash. 2d 662, 669, 732 P.2d 989 (1987) (quoting Illinois Cent. R.R. v. Illinois, 146 U.S. 387, 453, 13 S. Ct. 110, 36 L. Ed. 1018 (1892), aff'd, 154 U.S. 225, 14 S. Ct. 1015, 38 L. Ed. 971 (1894)). Due to the "universally recognized need to protect public access to and use of such unique resources as navigable waters, beds, and adjacent lands," courts review legislation under the public trust doctrine with a heightened degree of judicial scrutiny, "as if they were measuring that legislation against constitutional protections." Johnson, supra, at 525, 526-27.
This court did not expressly adopt the public trust doctrine until 1987, but indicated then that the doctrine has always existed [*28] in Washington law. See Caminiti, 107 Wash. 2d at 669-70. The doctrine in Washington "prohibits the State from disposing of its interest in the waters of the state in such a way that the public's right of access is substantially impaired, unless the action promotes the overall interests of the public." Rettkowski v. Department of Ecology, 122 Wash. 2d 219, 232, 858 P.2d 232 (1993).
The test of whether or not an exercise of legislative power with respect to tidelands and shorelands violates the 'public trust doctrine' is found in the following language of the United States Supreme Court:
The control of the State for the purposes of the trust can never be lost, except as to such parcels as are used in promoting the interests of the public therein, or can be disposed of without any substantial impairment of the public interest in the lands and waters remaining.

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http://www.ecy.wa.gov/pubs/93054.pdf
http://hometown.aol.com/rnwhiteley/trust2.htm http://law.utoledo.edu/LIGL/public_trust_doctrine.htm