Originally posted by Veseljko:

Swamps are really easy to spot from the distance.
And don’t bother to visit small strips of swamp biome between mountains and ocean. Those are mostly empty. Try to find a big swamp area where you don’t see anything behind. That will most likely be the only swamp you will need.

I would disagree with this. Small strips of swamp are where I consistently find my first turnip seeds on a new save and they are less likely to generate an abomination which you likely aren't yet ready to handle.