The third significant group to avoid being wiped out by the great wave did so simply by living well away from it. On a island large enough to contain multiple lakes near the mouth of the river that in another time and place would be called the Oder*, a few days walk away from the Lake Dwellers, is village of well over two thousand. Being so far from the Great Plain, at least by the standards of people mainly limited to walking speed, their ancestors were a bit behind those to the west of them in starting to cultivate crops. In fact them doing so was mainly the result of them not wanting to have to leave an excellent fishing and bird hunting spot. With a sea to the north of them, a small lagoon to their west , a large lagoon and river to their south and the lakes within the island itself, which aren't much use for fish but are magnets for waterfowl they have a never-ending abundant supply of protein. Having to put a bit of work into maintaining the supply of things to go with that protein seemed a very small price to pay for the advantages of staying put. The only reasons they haven't overtaken the Lake Dwellers in population are that they settled down a few centuries later, have had less people marrying in, and losses from past conflict with their neighbours.
The neighbours in question once included the Lake Dwellers. But when the great-grandparents of the great-grandparents of the current crops of elders were small children and infants, things got to a point where both sides realised that livestock raiding wasn't worth the effort of getting to each others turf and back home, not to mention the losses sustained, that they could call it more or less even regarding other grievances, and, most importantly, that they had other troublesome neighbours to deal with. So they hashed out a treaty of sorts that largely involved a river roughly halfway between the two of them and a general attitude of "you stay on your side, we'll stay on our side and we won't have any more trouble from each other so we only have to worry about those other assholes". Initially they just avoided each other but after enough decades had passed for past hostilities to fade out of living memory relations started to warm. Add in a fair amount of intermarriage and a lot of trading and by the time the plain was drowned things were bordering on being downright friendly.
The Sea Farers have a lot in common with the Lake Dwellers. Same religion, same culture, same language, same style of houses (although due to living by the sea those of the Sea Farers are somewhat better insulated and more solidly constructed), most of the same crops. But there are differences, mostly stemming from their different locations. Both use canoes and coracles, but the Sea Farers skew more towards the former than the later, have more of them and they're often bigger. As they get most of their protein from other sources, the Sea Farers' herds of deer are far smaller than those of the Lake Dwellers or Deer Herders, just slightly more than what's needed for draft animals and a milk supply for the smallest children. They've also made a few tentative additions to the crop package in the form of a few coastal plants deemed worth the effort. Sea kale provides both greens and large, starchy roots. Sea sandwort is a well liked green vegetable, although one more managed than cultivated, as is hastate orach which is actively cultivated. Sea buckthorn provides an abundance of berries even in winter, the Sea Farers' growing collective conviction that they prevent and cure a variety of winter ailments doing a lot to make up for the difficulty in harvesting them. And beach peas provide both greens and tasty seeds**. With regards to the last two, some of the Sea Farers have started to notice that plants planted near them or where beach peas were planted the previous year grow better than those elsewhere, but they haven't quite grasped the significance of that observation yet.
As far from the Great Plain as they are, the first learn on the flood from the Lake Dwellers a week or so after it happened. It would have been sooner but with everything else going on, mostly in terms of attending to the dead and coming to terms with what happened, the Lake Dwellers had other things on their minds than passing on the news. Already vaguely aware that "their" sea is separate from the one that flooded the plain, they conclude that the god of their sea is a far kinder or at least more reasonable one than that of the other sea, and possibly one who is fond of them. They already regarded it as fickle but generous, the usual hazards and difficulties of the sea being balanced by the abundant fish and the amber washed up on the shore, so this isn't too far of a stretch. They also reflect that the Lake Dwellers are closer to "their" sea than the plain.
When their conclusion as to why they were spared gets passed on to first the Lake Dwellers and then the Deer Herders, followed by the Deer Herders' theory getting passed onto them via the Lake Dwellers, a certain degree of theological cross pollination occurred, with interesting results that would have significant consequences down the line.
* The island in question is OTL Usedom on the German side of the Germany-Poland boarder.
** Beach peas have a reputation for being dangerous due to being one of the number of related plants where excessive consumption of the seeds causes Lathyrism, but the key words are "excessive consumption". They need to make up about 30% or more of the diet for a prolonged period of time before things get dicey. Them being a "sometimes food" that rarely clears the 10% mark is, as far as I can find out, perfectly safe. Also, from what I've read there's no danger in eating the leaves and shoots.