I hate to say this but beaches and sand and camera housings do not go well together.
I would not wash the interior of the housing and if I did it would be only with distilled water.
The O ring should be removed from the groove and the groove cleaned with a Q tip. The surfaces wiped with a micro fiber cloth and the port glass cleaned only after being rinsed of all sand with a micro fiber cloth, gently. The O ring can be cleaned by pulling it gently through a micro fiber cloth or your fingers and then lightly lubricated with your fingers.
If you have a large amount of fine sand in the control shafts and buttons, close the housing up, camera not installed, main O ring installed, immerse in clear water, running water helps, and pump, rotate, operate all controls over and over gently at first until the sand is removed. In some cases this is reason to require professional service.
DO NOT, I repeat, do not use soap on the housing. This can cause the seals to leak and is one reason why it is asked not to put masks with spit and anti-fog into the camera rinse bucket. The soap in the anti-fog will ruin the seals.
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