I accidentally spilled water on my cajon and am afraid the quality of sound produced by the cajon has been affected . I am wondering if there are any side effects to the cajon when water is spilled on it
I don't own a Cajón (even if I plan to buy one) but I'll try to answer your question.
A box made of wood
A Cajón is a box made of wood (most of them). Anything that can alter the wood can alter the sound.
The particularity of a Cajón is the front piece of wood which is thinner than the other sides, which allows elasticity and resonance. The front striking plate is generally made of plywood and is generally raw.
The wood thickness affects, of course, the sound. Any coating changes the sound also.
Damaged or not ?
That being said, your box is certainly partially varnished. But if certains pieces of wood absorbed some of the water and especially the front piece, the instrument is certaintly altered. If the front piece is damaged, this is a real problem. The other sides are somehow less important.
The thickness of the different wood parts is a hint (water causes expansion of wood). If so, the box is probably weakened and may break over time.
Also, if your instrument is essentially fixed with stainless steel screws (and not glue), and the water didn't damage the screw holes, it's ok. If not, the water may have altered the glue and your Cajón may split over time because of the shocks.
Fix it ?
The first thing I would do is to determine if the thicker sides are ok (left, right, top and bottom). If not, don't fix it, just change the whole instrument.
If only the front and back pieces are damaged and fixed with screws, you can change them : it is only wood, any DIY store will do the deal :)
Put your box in the sun to dry it up.
If you have cords/guitar strings in it, they are generally made of steel, check for any trace of rust and change them.
If it is an expensive Cajón, you can ask to a repairer in a percussion shop.