Ok so I tested several new seeded worlds. It's not a bug with old seeds it's just a very mixed bag with Mistlands. There are chunks with mostly land areas and there are areas with rock pillars jutting straight up from the ocean. If it were me I'd raise the land level 20 or 30 feet and back off a bit on the water but there are areas that are playable without swimming constatly. As long as you didn't explore the mistlands areas on your old original seed it seems you are fine. The land is as intended.