Is the Montjuic water fountain show time really all that spectacular? and-- on good Friday, how many minutes in advance would one need to arrive?
Judge for yourself: http://infocatalonia.eu/w/54XRj
On that day it's likely to be packed, so say 20-30' before. In any case, there are several 25' shows so if you can't make it for the first, there are two more. The difference between them is the sort of music to which the lights and fountains "dance", from classical to modern tunes. It's free and people stand around the fountain -or seat in the stairs. For info, it's rather casual, no assigned space or anything, just pop down and stand wherever you find a free spot. Btw, always watch your belongings when in a very crowded place like this.
Schedules: http://lameva.barcelona.cat/en/enjoy-it/magic-fountain/magic-fountains-show-times