Why would you swim between the two? A more common sense approach would be that the PLAN(People's Liberation Army Navy), has an actual "Bluewater" fleet in the Fallout Universe unlike the "Brownwater" capability it has in our universe. A massed "D-DAY" style landing is just more likely!
I might suggest, instead of wild suggestions like China can only invade America through the Bering Straight, that, when considering the Chinese attacked first, a huge surprise assault on Anchorage is plausible. In our own time, Chinese naval forces are slowly coming on par with American naval forces, even in some cases surpassing the restraints of American Carrier reliance (Mercenary's Mace and the advantage of the huge Chinese submarine fleet), especially in a surprise attack you wouldn't need that complicated a naval force. Without ports in Anchorage and wolf packs obviously roaming the American shoreline (Shi Empire), it would be a damnable experience to stop an invasion and that invasion holding onto their primary beachhead, which would mean a huge battle inland. When you count the naval confrontation near Alaska and the American fleet attacking China (Mercenary's Mace) in retaliation after Alaska was liberated would explain why much of the Pacific Fleet was gone.