I would absolutely not assume this creature meant me any harm

The first one, I would be wary because of the colors. I'd rather run from a corn snake than be bitten by a copperhead. When I see color like that, I'm taking a step back regardless, before I even have time to look at it's face/head

The second one... also gonna be wary of any swimming snake. Could be mostly harmless, or it could be a cotton mouth; but I'm not gonna check it out closely enough, before backing up. It still kinda has that "angry brow" too, if I did happen to get close enough

But I see your point either way. You can't rely on just one factor for identification. The snake in OP's video isn't a threat to me, so it's easier to observe.

I guess what I'm kinda saying, is that it helps to know a quick distinction between venomous/non-venomous. It might not be perfect for identification, but it helps the knee-jerk fear of thinking "the only good snake is a dead snake". It at least helped me reduce my own fear, when I moved to a place where venomous snakes are much more probable.

It doesn't keep me from reacting, but it definitely helps me from over-reacting

Edit: Looking at pics of the spine-bellied sea snake (Hardwicke's sea snake), that thing definitely seems deceiving! It really doesn't fit my usual assumptions. I still wouldn't mess with any swimming snake, unless I absolutely knew what it was... but still, that is different from what I've seen

They still have a bit of an "angry brow" and they are technically venomous... but I'll be damned if they ain't cute. One of the few snakes that make me feel compelled to snuggle them