Are you talking about that area on Seven Seas Lagoon across from Contemporary in this picture? I could maybe see there being room there, but then it couldn't be on the monorail line. Or are you talking about the weird shaped stretch of land South of the water bridge that connects Seven Seas Lagoon with Bay Lake?
The swamp on the land on Seven Seas Lagoon South of the water bridge is big and visible enough on this satellite photo, but they could have still done something with it. Have you ever been to Coronado Springs? They dug dirt out of the swamp to build the land around it and turned the swamp into a lake. If Disney really had their hearts set on building a resort there, they could have done something similar, or also pushed the land from the lake in to push the swamp water out. There is a lot they can do, so saying that land is "too swampy" is no excuse. It's a swamp, not an Everglade.
And out of all the Imagineering books I've read about Disney World and it's resorts, this is still the first I have ever heard of Disney having resorts planned for that plot of land or themed to the Venetians or the Mediterranean. And if they really wanted those themes, why didn't they build them elsewhere?