So during the Fourth Crusade, many sources indicate that there were large amounts of soldiers that left from alternative, more convenient ports, such as Marseilles and Flanders. Where did these Crusaders end up going? Did they end up in Venice, did they join the soldiers later at Constantinople, did they head to Egypt and the Holy Land, or did they simply disperse?
Short answer, since I have little info to go on:
Men from Flemings, led by John of Nesles, set sail from Flanders and took the English Channel down and around the Iberian peninsula. By the time they heard of the Fourth Crusade's disreputable diversions, they elected to sail straight to the Levant to join with and resupply the main Flemish force under Count Baldwin.
Burgundians and a miscellaneous assortment from Northern France took the Rhone river to the southern coast and from there hired ships out of Marseilles and Genoa. I can't find reference to their affairs afterwards, but they certainly didn't digress to the Adriatic.
By 1202 as the main forces assembled in Venice, a number of Frenchmen who weren't under the directives of the Crusade's leaders - including a renowned knight named Villain of Neuilly (yes, Villain! The etymological root only gained negative connotation in the last few hundred years) - crossed the Alps around Montferrat and Lombardy. At this point some of them were aware that the Crusade was going to struggle to reach the size estimated in the contract with Venice, and so decided to avoid the potential costs and obligations by instead continuing to the south of Italy, where they found more convenient passage to the Levant.
This early fragmentation of the forces may have been one of the key factors in the Crusade's eventual misfortune, as it put unexpected financial pressure on those who did meet in Venice and led to tensions with their hosts. The sack of Zara at the behest of the Venetians, condemned by the Pope, cane shortly after and was an ominous foreshadowing of their later violence against Christians.
Of course, such piecemeal assemblage and the utilization of varied routes had been typical of earlier crusades, even with the centralizing influence of royal leadership (something conspicuously absent from the forces at Venice).
As an aside, the plan to sail to Egypt had been decided in secret at Venice in the contract between Dandolo and the leaders of the Crusade. I'm not sure why it was kept secret since capturing Alexandria as a stepping-stone to Jerusalem had been proposed before, but I doubt any of the contingents that found their own paths east would've been aware of any intention to divert to the Nile Delta.
source: Johnathan Phillip's The Fourth Crusade