My focus as an archaeology student lies elsewhere geographically, however during some studies on maritime cultures and technologies I’ve touched briefly on Norse and Viking topics. So please N.B. other people will know better than me.

That said, it is my understanding that the typical “Viking funeral” known in popular thought - casting off the deceased on a longship, generally shooting a flaming arrow into the vessel for a vivid cremation at sea - is a modern fabrication. Off the top of my head, many images relating to the vikings (see horned helmets) come from 19th century fiction.

There is a common argument against this method that seems to fixate on the massive cost of such a burial type, however I don’t buy into that. Much more affluent burials have been attested through history (Egyptian pyramids, Mausoleum of Halicarnassus, Augustan family tomb, etc.) and even something as simple as a pure cremation can take on significant meaning as a form of conspicuous consumption (i.e. showing off power through destruction of goods; for example in mainland Greece good wood was sparse and the quantity needed to fully cremate a human would have been very resource and labour intensive).

More importantly, ship burials were very much a thing (Oseberg, for example), and often in lieu of an actual ship, a symbolic ship could be demarcated with stones, which would bear the same significance as a real ship in social reception.

It should be noted though that these were nonetheless rare. This is because, as many argue, ships are actually incredibly resource intensive - both in terms of physical resources and labour. The Sea Stallion of Glendalough, generally viewed as the absolute pinnacle of reconstructive archaeological reconstructive science, showed researchers that building a Viking ship would take something like 27,000 labour hours total. Considering the limitations of historical labour resources, building longships was thus quite a significant affair, and to simply destroy one or remove one from service for the sake of a destroying it when it’s otherwise perfectly functional might reasonably seem ludicrous. That’s probably why so few (relatively speaking) occurred.

Another thing is that all physical evidence we have for Viking ship burials come from burial mounds, or sometimes simply interred contexts (where the mound has been worn down), and this is a significant part of why (if I’m right in thinking sea burials are a modern fabrication) a ship burial on land is important: a burial mound built up on top of a full-size wooden Viking longship is a massive physical landmark. In creating it, not only have you benefited from the social spectacle of dragging a ship onto land and moving hundreds of tonnes of earth to cover it, besides simply the act of removing an incredibly valuable vessel from the sea; you’ve now also created a landmark that is, for all intents and purposes, a permanent reminder of the interred figure.

In an ancient world where mapping is mental and geography is landmark based, and the physical landscape defines your perception of the world, ‘Harald’s mound’ is going to have a very permanent status in collective social memory, much like the pyramids in Egypt. The function, really, is to immortalise your spirit in the minds of the people, because on death only your legacy will survive you. And, probably more importantly, the social capital of such a big event and landmark for a successor is likely immense, which is probably why we see the anthropologically interesting phenomenon of funerary display pretty much universally, at least in an Indo-European context.

So to answer your original question, did they ever wash up in foreign lands? Probably not - the floating burial you’re thinking of, and that most people think of, probably never happened. Maybe it did - maybe even in historically unknown cases - but tracing what happened to the ships if that was the case would be nearly impossible. If they did wash up, we have no historical accounts (so far as I know) from anywhere, and there would be no archaeological evidence save for in miraculous circumstances because essentially all the components of the ships would decay - the wood, iron nails, woollen sails, everything - and that’s assuming the ship made it to shore and didn’t sink. And if it did sink, then the location and environment it sank in would be a huge determinant in what did and didn’t survive. And we do have sunken Viking ships to help, but none to date have been associated with any kind of sea burial.

Hopefully this helps, and hopefully it’s not disappointing.