It could very well be that there is more than one person with a boat capable of going great distances across the ocean, plenty of things in the Fallout universe end up using some vauge, logical reasoning to peice together the back story, like how did the super mutants get to D.C if they started off in California?, or how come there is so many deathclaws in the wasteland, if they wer alegedley Bio-enguneered by the pre-war military?, all the questions usually lead to a vauge answer where you esentialy have a good a guess as anyone else, oh i just thought of another one, why is there only one ZAX computer in D.C?
Most of the technology presented in the Fallout series existed pre-war, including most of the weapons used. With how many weapons, armor, etc. I used to repair my equipment in Fallout 3, I found it hard to believe there would even be any pre-war weapons left. As for the bio-engineering by the military and scientists, it isn't so far fetched that while they were generally located in isolated facilities the knowledge could be passed on and spread to the military at other installations, as well as the government (Enclave). Since the Vaults were essentially one big experiment, mostly orchestrated by the Enclave, I would easily believe that one of the Vaults in the D.C. area could be designed with the sole intention of turning its residents into super mutants. Afterward, using whatever virus was left in the Vault, the super mutants could go out and begin capturing others to produce more. Deathclaws are produced by an egg-laying queen, like many insect societies. Since they are very tough, intelligent, and at least partially resistant to radiation (to the best of my knowledge), it is not surprising they would thrive in the wasteland.
Edit: And, as for the foreigners question... It isn't entirely implausible. As has been suggested, it is possible to use or manufacture boats for the trip. Another possibility is that further north there is a large amount of ice forming what would essentially be a bridge or solid ground that could be crossed to arrive in the Capital Wasteland. While such a thing might not exist year round, it could possibly last long enough for such a journey. It goes along the lines of 'nuclear winter.' Granted, so far from the nuclear war, any cooling effects to the planet should have subsided.