No real laws against them, unless of course some waters have no boats at all etc. I mean all it is is glorified wading to be honest and I don't know a whole lot of laws against wading 99% of the time. Most boat laws typically refer to engine size or gas/electric. I mean you'll have to pay attention to some waters that may have strict invasive species regs or something in which you aren't allowed to wade or your boat must be checked/cleaned etc. 99% of the waters you'll wanna fish you should have no issues.
I do it a lot and temp of water isn't an issue. Spring and fall I use my breathables, colder water I'll hit up my neoprene but usually June/July/August I go without waders completely when the air temps are hitting 80+.
I have done it in the salt..though that is a ton creepier lol..I am man enough to admit that. I usually do it in areas somewhat closed off to the main bay, channels, estuaries etc where I can keep safe from rips and tide changes. I wouldn't recommend open water obviously as there just inst enough power to fight a good current should you find your self in one. Anytime I've done salt I've always had at least one other person with me that was kayaking or canoeing as well, just as a safety net. A quick call to them and they could tow my arse if need be.