That's what it means: a vessel for carrying passengers across a not very wide stretch of water, in this case the bay of Cadiz, between Puerto de Santa María and Cadiz. The last one was the Adriano III, which sank in 2011:
http://www.diariodejerez.es/jerez/historia-barcos-delpasaje-...

"El pasaje" means the crossing between the two places.

"Ferry" make may modern British people think of the channel ferries, but it's quite an old word; it's in Shakespeare ("the common ferry / which trades to Venice"). If you want it to sound a bit more historical, you might use "ferryboat", or even "boat of passage", which is occasionally found in older texts, but perhaps sounds too small (though "ship of passage" would sound too large; it suggests an ocean-going vessel). On the whole, I think I'd go with "ferry".l

--------------------------------------------------
Note added at 31 mins (2018-05-21 20:01:01 GMT)
--------------------------------------------------

I can still remember the first time I went to Cadiz, by train from Madrid. It took about half an hour to go round the bay. I didn't realise there was a ferry.