As others have said, it would be helpful to know what parts challenged you. If you post the specific issues, we could run you through some sample problems.
Here is what to remember for what most consider the "hard" parts of the physics exam:
1. General gas law: (P1xV1)/T1 = (P2xV2)/T2. The general gas law is a combination of Charles' law, Guy-Lussac's law, and Boyle's law. To use the formula, substitute given pressures, volumes, and temperatures to derive an answer. If a variable is not given, then it is either (a) constant (meaning you can eliminate it from the equation) or (b) it is the variable you are trying to find.
2. Archimedes principle (an object is buoyed up by a force equal to the weight of the liquid displaced; objects of equal volume are buoyed up by an equal force). It's helpful to think of "up force" and "down force." The "up force" is the weight of water displaced by an object. The "down force" is the weight of the object. So if a question asks you how to make something positive, you calculate the amount of water to displace to give it a greater "up force" than "down force." Likewise, if a question asks you how to make something "X lbs" negative, you calculate the amount of weight you must add to increase the "down force" so it exceeds the "up force" by "X lbs."
3. Calculations using the general gas law are always expressed in absolute pressure. The surface is zero psi GAUGE, but 1 atmosphere absolute. A common error is to omit atmospheric pressure at the surface from calculations involving pressure at depth.
4. "Taking it to the Surface": When presented with a question that gives preliminary information at one depth, and then requests an answer for a variable at another depth, you must make an "intermediate" calculation of the condition of the variables at the surface before calculating the condition of the variables at a second given depth.
5. Gas consumption questions are just a variation on a theme. All you have to do is divide or multiply a given rate of consumption by absolute pressure. Remember to take it to the surface first.