What is it?

What a nasty piece of construction. Cringe-worthy deck?

rough in for gardening/potting sink

Fishing pole holder

I don't know what it is. I found it on Reddit where they were asking the same question. Given that there appears to be water under the deck, my guess would be fishing pole holder.

It might hold a cane pole but not something with a reel. Pretty sure it's for a flag or pennant.

@Dave Bair, Ah, good point.

Added later 1 h 16 min 48 s:

Here's another one from Reddit. Says he picked up at a thrift store, appears handmade, but isn't sure what it is used for.

That one is probably a book binders clamp, but would also work well for a vise to cut dovetails if attached to a bench.

Added later 05 min 03 s:

Moxon vise is the name I couldn't think of for the dovetail vise.

Added later 01 min 21 s:

Now that I think of it it probably is for woodworking with the dog holes.

I would agree that it is a fancy variant of a Moxon type vise to cut dovetails.  Especially since the outer clamping pad has a heavy chamfer to allowing one to cut blind dovetails without cutting into the pad.  My 2 cents.
  Ron