My new to me 2011 S releases quite a bit of water after a wash or rain when opening the tailgate. I'm worried this water is trapped somewhere it doesn't belong. Open the gate and water splashes on both rear fenders. Is this normal?

 

Without seeing a video - I'd say normalish. Some water always splashes on my rear fenders under those conditions - but nothing excessive IMHO.

Video of your concern might get a more accurate answer.

 

It will do that to some degree, but you could also have clogged drains.

Open the tail gate and on the bottom you'll find black plastic drains on each side. Pull those out and take a look. Even if things look pretty clean, pop the "valve" out and clean it well. I've found it doesn't take much to gum those up and that will cause it to collect water (and enough could backup and spill into the cabin).

When you put them back in, the spout on the drain is angled a bit and they should be positioned so that they point away from the cabin when the tail gate is down (e.g. pointing upward when the tail gate is open).

 

If he is experiencing what I've seen happen, it is a lot more than the normal amount and will actually splash into the cargo area a fair amount.

It's the rear passenger doors that irritate me the most for "normal" water. Opening either one of those when the car is wet always results in water getting splashed into the car