Yeah, resupply of the city will always be mainly by road(unless coming from the south by the river), but messengers aren't bound by such hassles, and what we're mainly concerned with is connectivity for admin purposes, which isn't supply heavy. So sea access isn't as important administratively, though it doesn't hurt.
The real issue is we simply can't afford to add all the rest of the stuff to make Redshore a proper True City to our pile of things to do in the near term, which means it's not a good idea to put the capital there, despite how handy sea access would be.