All watercraft come with a stock impeller, however this impeller is made to work efficiently for a wide variety of users and scenarios. The standard style of impeller is going to be more middle of the road performance-wise. If you are looking to get more top speed or bottom end out of your watercraft, this is when it is time to upgrade. You will want to take into account the following:

Weight- The more weight being carried, the more bottom end the watercraft will need to get on plane faster, therefore pitch would need to be reduced on the impeller to limit the volume passing through to the pump.
Performance- Depending on overall weight of the watercraft, rider experience, and performance modifications, a rider may need a more aggressive impeller to suit added power and performance.
Horsepower- It is important to note the overall load and top RPM the watercraft will have after tuning at the very least.
Altitude- This can play a major role in watercraft performance. Using an impeller that works at sea level will restrict the motor at greater altitudes, and vice versa.
Materials- leading brands in impeller manufacturing use stainless steel which allows for added strength and less weight than materials such as aluminum.
Pitch- Smaller pitches will improve acceleration and reduce top speed, while increasing pitch will decrease acceleration while improving top speed. Combining smaller pitch at the leading edge of the blade with a larger pitch at the trailing edge, also known as Progressive Pitch, allows the impeller to grab water and catapult it back towards the pump, essentially giving you the best of both pitch styles.
Blade Configuration- Having overlapping blades allows impellers to have increased blade area, and thus increases vacuum and allows the impeller to pull more water in towards the pump.