Hydrozoans are mostly marine, though a few are freshwater (Hydra).

Hydrozoans may be solitary or colonial. Most hydrozoan species have both a polyp and medusa stage, but others make use of only one or the other.

 

Other Hydrozoan is the Portuguese man-of-war (colony of polyps and medusa). The sting (nematocyst) from their tentacles is potentially dangerous to humans and often is lethal.

The Portuguese Man of War is often confused with a jellyfish, which is incorrect since it has no means of propulsion and is pushed by the winds and the current. The medusa form of Hydrozoans are known as hydromedusae.

Portuguese man-of-war
Source: Wikipedia

Freshwater and sessile hydra exists only as polyps

 

Hydra is a predator and eats small crustaceans, insect larvae, and annelid worms.