Asealed balloon filled with air that has a volume of 4 liters at 99 feet of seawater (30.2 m) will have a volume of liters at 33 feet of seawater (10.1 m).

We are given
V1 = 4L
d1 = 99 ft
d2 = 33 ft

We are asked for the volume, V2 of balloon at d2

Based on the gas laws and the weight of seawater, it has been established that at 33 ft of sea water, the pressure is 1 atm. When the sea level is doubled, the pressure is also doubled. So, pressure and sea level are directly proportional. From Boyles Law, pressure and volume are inversely proportional. In equation form:
P1/d1 = P2/d2
P1V1 = P2V2
So,
V1/d1 = V2/d2
Substituting the given values:
4L / 33 m = V2 / 99
V2 = 12 L
Notice that the volume is increased 3 times since the sea level was also increased 3 times from 33 ft to 99 ft.