boquear

Literally to gape or open the mouth repeatedly — most often used for someone gasping for breath in their final moments (está boqueando, he's drawing his last breaths) or, of a fish out of water, opening and closing its mouth. A secondary, more literary sense covers pronouncing or uttering a word.

• El pez boqueaba fuera del agua. — The fish was gasping, mouth opening and closing, out of the water.
• El anciano boqueaba, agotadas ya sus fuerzas. — The old man was gasping for breath, his strength spent.
• No boqueó ni una palabra en toda la reunión. — He didn't utter a single word the whole meeting.

Its most common modern use is specifically about someone dying or gasping for air — using it as a casual synonym for 'to speak' (hablar) would sound archaic or overly literary.