A state's public safety regulations are enforceable within its boundary.
Each state DOES have a specific surveyed boundary,
regardless of whether they flooded the river or not.
The regulations of a state apply within its boundaries only.
[Example: Commit murder while boating in the river on the California side,
and the Arizona LE official may hold you until the California LE official arrives.
You will be charged under California law if you were on the CA side of the boundary.]
Not exactly.
Arizona Revised statute 37-620.11. Enactment of compact
The interstate compact for jurisdiction on the Colorado river is enacted into law as follows:
Section I. Concurrent Jurisdiction
A. If conduct is prohibited by two adjoining party states, courts and law enforcement officers in either state who have jurisdiction over criminal offenses committed in a county where the Colorado river forms a common interstate boundary have concurrent jurisdiction to arrest, prosecute and try offenders for the prohibited conduct that is committed anywhere on the boundary water between the two states and concurrent jurisdiction to arrest offenders for prohibited conduct that is committed on any land mass within twenty-five air miles of the Colorado river or within twenty-five air miles of any lake that is formed by or is a part of the Colorado river.
B. This compact does not authorize:
1. Prosecution of any person for conduct which is lawful in the state where it was committed.
2. Any conduct prohibited by any party state.
Section II. Ratification
This compact is ratified by enactment of the language of this compact, or substantially similar language expressing the same purpose, by at least two states of which the Colorado river forms a common boundary.
California Penal Code
853.1. (a) Pursuant to the authority vested in this state by
Section 112 of Title 4 of the United States Code, the Legislature of
the State of California hereby ratifies the Colorado River Crime
Enforcement Compact as set forth in Section 853.2.
(b) The purpose of this compact is to promote the interests of
justice with regard to crimes committed on the Colorado River by
avoiding jurisdictional issues as to whether a criminal act sought to
be prosecuted was committed on one side or the other of the exact
boundary of the channel, and thus avoiding the risk that an offender
may go free on technical grounds because neither state is able to
establish that the offense was committed within its boundaries.
(c) This compact shall become operative when ratified by law in
the State of Arizona; and shall remain in full force and effect so
long as the provisions of this compact, as ratified by the State of
Arizona, remain substantively the same as the provisions of this
compact, as ratified by this section. This compact may be amended in
the same manner as is required for it to be ratified to become
operative.
853.2. (a) All courts and officers now or hereafter having and
exercising jurisdiction in any county which is now or may hereafter
be formed in any part of this state bordering upon the Colorado
River, or any lake formed by, or which is a part of, the Colorado
River, shall have and exercise jurisdiction in all criminal cases
upon those waters concurrently with the courts of and officers of the
State of Arizona, so far and to the extent that any of these bodies
of water form a common boundary between this state and the State of
Arizona. In addition, the officers shall have concurrent jurisdiction
with the officers of the State of Arizona on any land mass within 25
air miles of the Colorado River, or within 25 air miles of any lake
formed by, or that is a part of, the Colorado River.
(b) This section applies only to those crimes which are
established in common between the States of Arizona and California;
and an acquittal or conviction and sentence by one state shall bar a
prosecution for the same act or omission by the other.
(c) This compact shall not be construed to bar the enforcement of
the penal laws of either state not established in common with the
other, provided that the act or omission proscribed occurs on that
state's side of the river channel boundary.
(d) This compact does not apply to Division 3.5 (commencing with
Section 9840) of the Vehicle Code, relating to registration of
vessels, or to Section 658.7 of the Harbors and Navigation Code,
relating to the display of a ski flag.
The reason why it became an issue about fishing is because the OP was about fishing, and the state officers will almost certainly be making the contact to check fishing license status and THEN will find the weapon issue.
Please note that these respective states have included the lands within 25 miles on either side of the shore.
)
Now off the coast of Cali, fishing for Sharks, hey I'd want my gun. 
