If you are having cavitation problems then you have a couple options.
1. If you stick with your current prop then you need to lower the motor. I would go down in 1" increments until you stop cavitating. Its all right to cavitate some as your motor increases in RPMs but once you are fully on plane the prop should catch and you should be able to hit a good top speed.
2. Assuming you have a manual jack plate then get a prop with some cupping in it. What pitch is to recommend is a little more difficult. but 11 -12 is good start. Go with 11 and medium cupping or 12 with mild cupping. Drop by one pitch if you go with a four blade prop instead of 3 blades.
3. If you have an electric TSG and your goal is to run shallow then you can usually go with a little more pitch and cupping. The reason for this is that you will be able to jack up higher once you are on plane. The down side is that you lose some hole shot.
If you want to be more specific about what is the exact prop then you will need to install a tach for your motor. I run a 25 Yamaha 2 stroke too. With a powered jack plate I am able to run a 12 pitch prop with heavy cupping.