Grew up working on awnings at the family business. Always followed sunbrella's recommendations:
General or light cleaning
To clean Sunbrella while still on a boat, follow these simple steps:
1. Brush off loose dirt.
2. Hose down.
3. Prepare a cleaning solution of water and mild detergent such as Woolite or Dawn dishwashing liquid.
4. Use a soft bristle brush to clean.
5. Allow cleaning solution to soak into the fabric.
6. Rinse thoroughly until all detergent residue is removed.
7. Air dry.
8. May not require re-treatment depending on the age of the fabric.
Also, 303 is recommended to retreat the fabric.