If there was Northern fleet for Robb would it be a game changer?

brenobac:

racefortheironthrone:

Depends on which coast the fleet was on. 

Maybe on both because he could bother the Lannisters at Lannisport and directly hit KL 

Well, the more consequential shift would be on the west coast, but more having to do with the Ironborn than Lannisport – if Robb could have forestalled the invasion of the North, he changes the outcome of the war significantly, whereas Robb was able to threaten the Westerlands sufficiently to make Tywin march west without a fleet. 

Over on the east coast, Wyman Manderly was building Robb a fleet in OTL, so probably less of a change.