How do you say pool in German?

If you are referring to a swimming pool then German translations are:

Schwimmbad

Schwimmbecken

Freibad

Hallenbad

It can also, depending on context, be translated as

Lache, Pfütze, Tümpel (pool of water)

Spieleinsatz (Poker pool)

Ansammlung

Pool, Poolbilliard (game of pool)

As a verb:

vereinen

zusammenschließen

ansammeln

zusammenlegen

bündeln