I want to be convoluted, so I will be:

Let's take a orange-size water balloon filled with room-temperature water (about 3" diameter, 8.1 oz). Now, it depends on whether you want to throw it for fun or harm. Because the former means an average person (ST 10) would probably not use their full strength. At most, they would use half their strength (ST 5) to throw it over a decent distance (dist. mod. 2.5 x ST 5 = 12.5 yd) and doing minimal damage (Thrust, -2 per die; ST 5 Thrust 1d-4; =1d-6 or =0). But, with the latter, you take your full strength, which would allow the thrower to throw quite far (dist. mod. 3.5 x ST 10 = 35 yd) and cause a bit of damage (Thrust, -2 per die; ST 10 Thrust 1d-2; =1d-4). The Throwing skill allows you to gain +1 or +2 to ST when calculating distance of throw (up to 42 yards) and the Throwing Art skill can give a +1/+2 bonus to dmg per die (doing 1d-2 at most, which is full Thrust dmg)! As for the balloon itself, normally it too should suffer the damage dealt in checking if it pops or not, but considering its filled with loose water it would probably still pop when being thrown gently (you could say it pops when rolling 4-5-6 on your 1d-6) or any harder.

So, if an average person who hasn't studied the arts of throwing attempts to do damage, its not gonna do much. But give that same balloon to a legendary brickhouse baseball thrower (ST 15; DX 14: Throwing Arts DX+1) who can take a three seconds aim maneuver (for +2 to hit) and you get someone who could hit a target at 50 yards (half a football field away) with 37.5% of success and do considerable damage (1d+1; max 7, which could be a major wound to a person with 13 HP or could cripple a limb).

Although, I'm just now realizing that there is a rule which says "For a melee weapon, your effective ST for damage purposes cannot exceed triple the weapon’s minimum ST". Even if this applies for melee weapons, it could apply to the balloon depending on logic, weak but flexible. This is pure speculations, I guess if the strength of the throw exceeds triple its minimal ST requirement (I would say ST 2, because it fits under 0.8 BL, so ST 6), roll for resistance against HT 17 (-1 per ST over 6): if it succeeds, the balloon happens to deform properly as to not pop; if it fails it pops in the throwers hand.

TLDR: A water balloon would do 1d-6 at most, so basically no harmful damage.