Is a whistle a day signal? Or if not, what would be a day signal?
Is the 16' and over day and night signal a Washington Dtate requirement, or a Federal requirement?
Oh dear, I hope I don't turn into WCP's safety rules lawyer! I hate everything about this stuff!
A whistle is neither a day signal nor a night signal. It is, instead, a sound signal. Per the
chart from Washington State parks, a sound signal is required of all boats, in all waters, at all times. The requirement to carry a sound signal is in addition to any requirement for day signals (properly, daytime visual distress signals) and, in some circumstances, night signals (nighttime visual distress signals). A list of visual distress signals—day, night, and both—is at 33 CFR § 175.310.
The requirement to carry sound signals and visual signals arises out of both Washington law (WAC 352-60-040, -065) and federal law (33 CFR §§ 83.33(b) and 175.110). However, either or both of Washington and federal law may set different requirements based on length of boat, time of day, and whether the boat is in "coastal waters." In addition, the application of federal law may be limited depending on whether the boat is within the Coast Guard's jurisdictional waters, defined
here for the Pacific Northwest. (But notice the incredible list of caveats at the beginning—the Coast Guard itself can't say exactly where its jurisdiction lies!) You cannot derive simple, universal rules. The rules vary depending on the circumstances.
As for your situation, you obviously can't extrapolate Washington state law to California, and you may not even be able to extrapolate the application of federal law in Washington to California because the Coast Guard's jurisdictional waters may be different in California than in Washington.
If you care about strict compliance with this stuff, then just follow the chart when you are in Washington, bearing in mind that the chart is misleading about night signals for kayaks under 16 feet. (Short kayaks only need to carry night signals when they are in coastal waters at night.) If you need to know California rules, you'll need to look for a similar, credibly-sourced chart that applies down there, or you will have to actually read California law and federal law and figure out for yourself what rules apply.
Alex