What is greek word for fierce swimmer?

There isn't a single, perfect Greek word to describe a "fierce swimmer." Greek doesn't work with compound words in quite the same way English does. You'd need to combine words or use descriptive phrases.

Here are some options, depending on the nuance you want to convey:

* ἀγρίως κολυμβῶν (agríōs kolymbōn): This literally translates to "fiercely swimming" – it's a participle, describing someone who is swimming fiercely. This is probably the closest single option.

* θαρραλέος κολυμβητής (tharraléos kolymbētēs): This means "bold swimmer" or "courageous swimmer." While not directly "fierce," it implies a fearless approach.

* ἀνδρεῖος κολυμβητής (andreios kolymbētēs): This translates to "manly swimmer" or "brave swimmer." Again, it captures the spirit of fierceness through strength and bravery.

To best capture the idea of a "fierce swimmer" you might need a longer descriptive phrase incorporating adjectives like δεινός (deinós - terrible, powerful, skillful) or ἀκάματος (akámatos - tireless) alongside κολυμβητής (kolymbētēs - swimmer). For example: δεινὸς καὶ ἀκάματος κολυμβητής (deinós kai akámatos kolymbētēs) - "a terrible and tireless swimmer."