The word remaban is the imperfect form of remar in the third person plural.
See the full remar conjugation

remar

Los remeros remaban al unísono con brazadas lentas.
The oarsmen rowed in unison with slow strokes.
Los galeotes cantaban para mantener el ritmo mientras remaban.
The galley slaves sang to keep the rhythm while rowing.
Estos ex pescadores remaban con fuerza y eran expertos, pero ésta era una de las peores tormentas de viento que jamás habían encontrado.
These ex-fishermen were strong and experienced oarsmen, but this was one of the worst gales they had ever encountered.
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