A little more context would be useful to identify the target audience, but it seems that "marejada" is usually (not by everyone)used in a technical sense for a "certain degree" of swell, whereas the term "swell" is supposed to refer to the origin of the undulating waves; although it is used by boaters to speak simply of gentle undulating (rounded) waves. Look at the varying definitions for "swell" (and Spanish equivalents)below, according to whether you are an oceanographer or a sailboat skipper(URLs with text):

Swell: Wind-generated waves that have traveled out of their source region, usually over a considerable distance. Swell waves exhibit a more regular and longer period with flatter crests than choppy, locally generated wind waves. (Glossary!)
http://www.ripcurrents.noaa.gov/glossary.shtml

Large smooth waves that do not crest. Swells are formed by wind action over a long distance.
www.terrax.org/sailing/glossary/gs.aspx

ongoing series of wind-created undulating waves on the ocean surface; the difference in surface height between the lowest and the highest point of the wave as it travels beneath an object.
www.ocean-institute.org/edu_programs/materials/P/Glo/S_Glos...

NOW, look at the following definition in Spanish (retrieved from Eurodicautom)
"Movimiento ondulatorio de las olas que persiste después de cesar la causa que lo originó. Puede ser causado por la existencia de viento a cierta distancia del lugar considerado, o por un viento que sopló durante cierto tiempo con anterioridad a la observación del fenómeno."
Reference P. O'Dogherty, CEH
(1) TERM mar de fondo
Reference P. O'Dogherty, CEH
(2) TERM mar tendida
Reference P. O'Dogherty, CEH

Now at the Spanish dictionary definition of "marejada" 1 Movimiento fuerte de grandes olas en el mar, sin llegar a ser temporal: pronosticaron marejada en el tercio norte.
s.f./ METEOROLOGÍA/ =marola
Gran Diccionario de la Lengua Española © Spes Editorial 2001

...and the following pdf(URL in text)from the international Commission for the Conservation of Antarctic Marine Living has definitions for Resources has a good set of explanations for the terms in Spanish (see page 10). This was my source for the least technical of my offerings, "oleaje mixto"
http://www.ccamlr.org/pu/S/pubs/s-obsmanp2.pdf

The same commission has a completely multilingual website at which can be mined for more data at:
http://www.ccamlr.org/

Note, however, that there is a Colombian technical website which could allow you to draw varying conclusions about "marejada":
http://www.cioh.org.co/proserv/dat_generales.htm


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Note added at 2005-12-24 15:50:49 (GMT)
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There is a nice summary of the Douglas scale in Spanish at:
http://www.diccionario-nautico.com.ar/escala-de-douglas.html