The question of what happened to John's baggage in October 1216, and whether it was actually lost at all, remains a pretty controversial topic among historians of his reign, but the most straightforward answer to your question is that the map of East Anglia has indeed changed pretty substantially since the early 13th century, when the area was still one extensive fenlands and coastal salt marsh.

Significant drainage work has been carried out across the Fenlands since that date, with the result that the shoreline of the Wash (a name probably derived from an Old English word meaning "ooze") has moved between 3 and 6 miles to the north-east, making a crossing of at least part of the area – historians seem pretty much agreed that the route concerned crossed only the delta of the Wellstream river, not the full body of the Wash itself – seem a much more attractive idea in 1216 than it is now.

There's more to the story than this, however. Three chronicle accounts (two of which are contemporary, or very nearly so) allege that John lost a vast quantity of treasure in the Wash in 1216 – Roger of Wendover, the second earliest, explicitly says that the king was deprived of "the treasures, precious vessels, and all the other things which he cherished with special care," and certainly when inventories of John's treasure in March 1216 are tallied with that in the possession of his son Henry III in 1220 there is very little crossover between the two lists, and some very precious assets, including the whole of the imperial regalia of John's grandmother, the Empress Mathilda, are never heard of again after 1216.

Nonetheless, our very earliest source, Ralph of Coggeshall, who is generally regarded as a more trustworthy writer than Roger was, says the only items lost were the king's collection of holy relics and some "divers household effects". If this is correct, then the treasure may in fact have been dispersed at another date. Ralph himself talks of speaking to a priest who went to Newark after the king's death there, who had seen men leaving the town laden with loot.

Sources

H.C. Derby, The Medieval Fenland (1940)

W.L. Warren, King John (1978)

Thanks that's very interesting. I'm afraid I was looking at a modern day map and it's obvious in hindsight that yes the geography would have changed just look at Dunwich. Thanks very much for giving me such a detailed explanation.