Provide a little more information.
What type of boat? Which engine?
Is the alternator stock, or did you upgrade?
Did you change any of the wiring with respect to the alternator/battery?
Do you have big ass amps on the stereo?
When you say the breaker trips when you go fast, how fast?
Is it a function of speed, time, or hitting wakes hard?
Here is something to try that would help identify the problem: Put the boat in neutral, but rev it up to the same RPM that you would be going when the breaker trips. Let it run there for as long as it would take to trip the breaker if you were speeding across the lake.
Did the breaker trip?
If yes, then somehow the high engine RPMs is causing excessive current to flow throught the breaker. One way this could happen is if the alternator output was connected on the "load" side of the breaker instead of the "battery" side. I have never seen a boat wired this way from the factory but I have seen a lot of people make this mistake when they upgrade their alternator.
Another "pot shot" at fixing the problem would be to simply replace the breaker. They do go bad occasionally, and a typical failure is for them to get weak and start tripping at too low a current. High heat will increase the tendency for the breaker to trip, as will vibration.
If your breaker is mounted on the engine (a typical spot) then perhaps the high speed run is causing a lot of heat from the engine, along with a higher frequency vibration to make a weak breaker trip. Just replacing the breaker with a new one might solve the problem.
What confuses me though is the statement: "When i come to a stop my stero will play.When i turn the key to ass the stero won't play"
By "come to a stop" do you mean you are at idle, but with the engine still running, or can you actually shut the engine off and still play the stereo?
If you mean you are at idle but the engine is still running then it supports the theroy that the alternator is wired to the load side of the breaker instead of the battery side. Once the breaker trips, the battery is disconnected from the circuit but the alternator is still providing the necessary power. As soon as you shut the engine off, however, there is no longer a source of power until you reset the breaker.
If this is the case, then you really want to rewire the alternator so that it connects to the battery side of the breaker. The normal wiring of the alternator has a fairly good size wire that connects down to the starter relay or the starter itself.
When the alternator is wired to the load side of the breaker you are forcing all the charging current to pass through the breaker on its way to the battery. The alternator provides its maximum output at high RPMs so the high speed runs is when the alternator might be cranking its full rated load.
If the breaker is rated at 30 amps, and the alternator is pumping 50 amps into the battery, the breaker is going to trip.