#### Answer

$758 mL$ is the volume of the balloon at a depth of 25 m underwater.

#### Work Step by Step

Using the combined gas law with the involved quantities we can see that
$\frac{P_{1}V_{1}}{T_{1}}=\frac{P_{2}V_{2}}{T_{2}}$
Since the question asks for the final volume, we must solve for $V_{2}$. For this question we must also convert the two temperatures into the standard units of Kelvin by simply adding 273.15 to the Celsius values.
$18^{\circ}C + 273.15 = 291.15 K$
$34^{\circ}C + 273.15 = 307.15 K$
Rearranging for $V_{2}$, we get:
$\frac{P_{1}V_{1}T_{2}}{T_{1}P_{2}}=V_{2}.$ Therefore, $\frac{1.0 atm\times 2.8 L \times 291.15 K}{307.15 K\times 3.5 atm}=V_{2}.$
$V_{2} = 0.758 L$
$V_{2} = 0.758 L \times 1000 = 758 mL$