der Segler
sailor
A person who practices the sport of sailing or sails a sailboat as a hobby.
Der erfahrene Segler kennt die Küste wie seine Westentasche.
The experienced sailor knows the coast like the back of his hand.

sailor / sailboat / swift
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sailor
A person who practices the sport of sailing or sails a sailboat as a hobby.
Der erfahrene Segler kennt die Küste wie seine Westentasche.
The experienced sailor knows the coast like the back of his hand.

The word 'Segler' is derived from the verb 'segeln' (to sail). The ending '-er' is often attached to verbs to refer to a person or device that performs that action.
Wer segeln lernen will, sollte einen Kurs besuchen.
Anyone who wants to learn to sail should take a course.
'Der Segler' (the sailor / sailboat / swift) belongs to a word family with the verb 'segeln' (to sail) and the noun 'das Segel' (the sail). All the words relate to movement by wind power.
Ohne Wind kann man mit einem Segelboot nicht segeln.
Without wind, you can't sail with a sailboat.
The context makes clear which meaning is intended. If people are talking about sports or persons, it means the person. If it is about harbors or the sea, it is often the boat. In connection with the sky or birds, it refers to the bird species.
Der junge Segler gewann die Regatta.
The young sailor won the regatta.
When 'Segler' means a boat, typical verbs are 'im Hafen liegen' (to be in the harbor), 'vor Anker gehen' (to anchor), or 'auf See sein' (to be at sea). It is also often combined with adjectives that describe the boat.
Ein majestätischer alter Segler lag im Hafen.
A majestic old sailboat lay in the harbor.
sailor
'Seemann' and 'Segler' can both describe people who are out on the water. 'Seemann' means a person who works professionally at sea on a ship, often as part of a crew. 'Segler' means a person who sails a sailboat, especially as a sport or hobby. Learners may confuse the words because both have to do with ships and the sea. The difference is mainly about profession and type of boat: a 'Seemann' works on ships, while a 'Segler' sails with sails, often in their free time.

Nach der Arbeit trainiert der Segler auf dem See für den Wettkampf.
After work, the sailor trains on the lake for the competition.
Der alte Seemann erzählt gern von seinen Reisen über den Atlantik.
The old sailor likes telling stories about his voyages across the Atlantic.