I guess let's pick apart each one:

Ocean water - idk how the release would play out, but I'm picturing an opening big enough for a person to fit through, like a manhole, releasing as a stream. While a lot, I don't think that'd be near enough to cause a hydroplaning situation, but it'd DEFINITELY get them off my ass, so win. Assuming the highest potential for harm here, all 3m³ released at the same time, yeah that could fuck up their trajectory in a way that sends them upsided down into a ditch... but if they're tailgating, they're already putting my safety in jeopardy, so they've crossed the line into me not caring if the solution does the same to theirs. Closest thing I've done IRL was spotting a piece of debris (chunk of a bumper or something from another car) on the road ahead while being tailgated, and I waited until the VERY last second to dodge it - the dude tailgating didn't have time to react, so he drove right into it. Made a really satisfying crunch, and he pulled over presumably to check for damages. Lost sight of him shortly after, since I just kept going. Could it have hurt or even killed him? Yeah, if he turned sharp and started rolling or something. Could his driving habits have seriously hurt or killed me? Also yes, so fuck him: my goal is to get him off my ass, not baby him.

Locust - Not seeing the potential for harm here. The religious nuts would be on high alert watching for fireballs coming down from the sky or rivers to turn to blood; but locusts are just big grasshoppers - not like they're going to start hunting people down. Seeing that hysteria unfold would be great fun!

Hornets - Kinda same spiel as the driving bit. You've probably seen that response to the paradox of tolerance, if not clicky. We're talking about a group of people whose goal it is to harm/kill me, my family, my neighbors, etc. And since that's the case, I don't owe them protection from harm. Hornet it up.