I have a theory I am working on, looking for the right world conditions.

- I've had serpents spawn extremely close to shore to kill leeches that moved from the swamp to the shores of a dark forest. As with most biome critters, serpents hate critters from other biomes. Many of my outposts are set on borders so mobs are distracted and kill each other. Having the bench/fire attracts. I make a large square ditch and let the action play out.

- My theory is that if you manage to find a stripe of land in the swamp that is adjacent to or just designated ocean you can raise the ground to a working level and set a workbench/fire. You may be able to prompt spawn conditions by abusing the aggro system and "baiting" with leeches. There was no reason whatsoever for my serpent to come to shore apart from killing leeches. If your world hates you as much as mine hates me, you will get hordes of leeches chomping at the shoreline. Let em build in numbers and wait n see what comes.