Concrete doesn't "dry," it hardens. It's a chemical reaction. Concrete will
harden under water.

The ROMANS used to make docks out of concrete by dumping it into forms below
the water - that was 2,000 years ago and the pilings are still there.

Well, it's not QUITE that easy. A hard driving rain can mess up the
top layer of concrete by washing away the dust, and mechanically
sorting the aggregate. Just plain water isn't a problem, but
MOVING water is. It's too late now anyway, I'd have put straw or
rags over the concrete.

Likewise, when pouring concrete under water, you can't just sink a
box in the bay and shovel conrete into it, you have to feed it
through a pipe, with the bottom end burried in the pour, so that
you don't get little blobs of rocks with layers of water between
them, glued together by what little cement didn't drift away as
silt on the way down.

--Goedjn