Compressibility of water
The old "hard as concrete" saying has much to do with the fact that water resists compression very well. Thus, if the jar hits the water (at less than terminal velocity, even if the glass is quite thick), the water will not compress enough to provide much cushion, so the glass will shatter. At slower speeds, the effects of breaking surface tension (think: water "getting out of the way" or making a splash) would allow the jar to splash into the water.
Dioxin liquid
Unfortunately, your specification of "liquid dioxin" creates a slight problem:
I don't know which dioxin you are referring to, but TCDD is a commonly known one, so I'll go with that.
TCDD is solid at room temperature, and unfortunately not very soluble in water ($0.2 \mu g / L $), which only allows for $4\mu g$ of dioxin in your 20L delivery container, which probably wouldn't kill anyone by the time it's further diluted in the target body of water. (Lethal dose in rodents varies between $1-1000 \mu g/kg$).
To wit, it would probably take in excess of twenty such canisters to kill a single human-mass monster and that's if they somehow managed to ingest all of the toxin from every jar, which is very, very unlikely given the delivery method. Clearly, they'll need the concentrated solid.
If you do have TCDD in a concentrated solid form, I'd suggest (er, in your fictional world!) to simply drop that. When it hits the water it will slowly dissolve, but would have a better chance of staying concentrated enough to do some damage.
Dioxin solid
TCDD is a crystalline solid. I've never seen it (and hope I never do!) but it would probably lend itself to small-ish crystals, granules, or ground up powder (the finer the grind, the more readily it would dissolve). To continue with the "jar" delivery method, I'd fill up the 20L jar with the powder, and then flood it with water. The water won't dissolve more than a few micrograms of the powder (see above), but it will give your package more heft and make it more likely to shatter the glass on impact.
Water hammer design
If you include an air gap so the liquid/solid combination can move inside the jar, you can achieve a "water hammer" effect to help break the glass:
The idea is that in between the top (poison) layer and the air (better yet, partial vacuum) compartment at the bottom, there's a relatively thin barrier that will break easily when the jar hits the water, which will cause the poison to accelerate and rapidly hammer the bell at the bottom and blow it out. The fins of course keep the vessel on a stable orientation during free-fall.
The concave bell shape on the bottom might help transmit more of the shock to the glass, but it's certainly not essential. You could use ordinarily-shaped jars.
While you don't need this kind of design, it's still simple, and it would improve the reliability of your characters' poison bombs and/or allow them to drop them from lower heights.
How high?
Not very.
With the above design, and 20kg of accelerating poison to help break the jar, a drop of a few meters on relatively calm water would suffice. Thus, your characters will want to handle them carefully, which would include careful packaging and carrying protocols (not to mention hazard pay...).
At drop distances of even tens of meters (30 feet), the jar will be moving fast enough to virtually guarantee the destruction of the vessel.
Bonus: How about explosives?
For even greater reliability (or use in cases where free-fall delivery may be unavailable), simply include a low-yield "slow" explosive (modern "smokeless" gunpowder would be sufficient, since it contains its own oxidizer). It needs only break the glass, so a tiny amount will suffice (or just use a bullet, if your characters have modern ammunition). Almost any kind of detonation trigger will do (contact, altitude, even a timer/fuse), since you don't care when it blows up, as long as the good guys aren't holding it at the time, right?
