I looked up the code for you and it turns out that the violation is a misdemeanor, not an infraction. Either the officer made a mistake, you heard incorrectly, or he lied to make you go away. The DA probably did not upgrade to a misdemeanor, it is one.
655.2. (a) Every owner, operator, or person in command of any
vessel propelled by machinery is guilty of a misdemeanor who uses it,
or permits it to be used, at a speed in excess of five miles per
hour in any portion of the following areas not otherwise regulated by
local rules and regulations:
(1) Within 100 feet of any person who is engaged in the act of
bathing. A person engaged in the sport of water skiing shall not be
considered as engaged in the act of bathing for the purposes of this
section.
(2) Within 200 feet of any of the following:
(A) A beach frequented by bathers.
(B) A swimming float, diving platform, or lifeline.
(C) A way or landing float to which boats are made fast or which
is being used for the embarkation or discharge of passengers.
(b) This section does not apply to vessels engaged in direct law
enforcement activities which are displaying the lights prescribed by
Section 652.5. Those vessels are also exempt from any locally
imposed speed regulation adopted pursuant to Section 660.