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 an "actual" 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