bathing, paddling (BrE), swimming, wading (AmE)
big
little, small
deep
empty
indoor
open-air (BrE), outdoor
heated
backyard (AmE)
in-ground (AmE)
Olympic, Olympic-size
baby (esp. AmE), children's, kiddie (esp. AmE)
inflatable
We gave the children an inflatable pool for Christmas.

lap (AmE)
a 25 m lap pool

diving (esp. BrE)
I left the diving pool after my session on the springboards.

wave
a unique new wave pool

plunge
a sauna and plunge pool

birthing (BrE)
babies born in birthing pools

swim in
dive into, jump into, plunge into
fill
drain
You won't have to drain the pool so often.

deck (esp. AmE)
I slipped on the wet pool deck and fell down.

attendant (esp. BrE), boy (AmE)
Jack works there as a pool boy.

party
A girl invited me over to her pool party.

the bottom, edge, middle, side, etc. of the pool
a length of the pool
He swam three lengths of the pool.

big, large
little, small
bottomless, deep
shallow
stagnant, still
muddy
Rhinos are fond of wallowing in muddy pools and sandy riverbeds.

freshwater, saltwater
rock (BrE), tide (AmE)
ornamental, reflecting (esp. AmE)

in a/the pool
The children waded in the shallow rock pools.

pool of
pools of water

big
little, small
dark
mud
The park is full of hot water springs and bubbling mud pools.

lie in
The body lay in a dark pool of blood.

in a/the pool
The wick floated in a pool of oil.

pool of
pools of light

big, huge, large, vast
a vast pool of knowledge on best practices

limited, small
They will share a limited pool of money with other defendants.

growing, shrinking
a growing pool of data
a shrinking pool of assets

available
the available pool of really good people

common
They draw on funds from a common pool.

diverse
this nation's diverse talent pool

car, labour/labor, motor (esp. AmE), typing
applicant (AmE), recruiting (esp. AmE)
We need to increase our applicant pool.

talent
There's a great talent pool in the business community.

jury (AmE)
press (esp. AmE)
A member of the press pool shouted out a question to the president.

gene
They share a common gene pool.

build, create, form
broaden, expand, increase
Supporting a diverse work force broadens our pool of talent.
a method of expanding the pool of potential investors

pool of
a strong pool of talent in the company
a large pool of cheap labour/labor