I plan to write a novel set on old Venus. What I mean by this is that it is set on a Venus where it was still thought to be a swampy planet. I know that is an unlikely past, but I have read oceans may have once existed on Venus but were probably lost to a runaway greenhouse effect. So reality check: is it plausible that Venus was once a swampy planet reasonably habitable to Earth like beings?
Since this is a science-fiction story set in an alternate reality, yes you can write about a swampy Venus. Firstly, this is an established concept in previous generations of science-fiction, and in the same way steampunk fiction harks back to earlier versions of speculative fiction of the kind written by Verne and Wells, there is no reason why you can't use swamp-infested Venus.
A recent anthology of swamp Venus stories titled Old Venus, edited by George RR Martin & Gardner Dozois, was published in 2015. So this can be science-fiction subgenre in its own right. There is no reason that makes sense to restrict writers to writing about a scientifically accurate Hell planet Venus. Science-fiction is a literature of the imagination as much as reason and speculation about possible realities.
If you want to add scientific authenticity to your swamp planet Venus either the Sun needs to be cooler, which is plausible because it was a cooler star in the past, or perhaps further from the Sun or a combination of both. This will keep Venus cool so there isn't a runaway greenhouse effect. The drawback is that Earth will be colder. Possibly, permanently glaciated with a cooler Sun. You can cheat by giving Earth more active and productive sources of methane to keep its global temperature up and at a level where our biosphere or a sufficiently close version of it can exist.
Normally I'd advise a science-fiction writer ignore being too pedantic about the scientific accuracy and just allow Venus to be a swamp world and leave it at that. Just go for it and write the best swamp Venus story you can.
The scientific community doesn't know for sure, but it is certainly possible, even plausible, that the planet supported liquid water.
What isn't clear is whether this water survived long enough for life to develop into the complexity necessary for a swampy surface.
Through studies of the present cloud structure and geology of the surface, combined with the fact that the luminosity of the Sun has increased by 25% since around 3.8 billion years ago, it is thought that the atmosphere of Venus up to around 4 billion years ago was more like that of Earth with liquid water on the surface. The runaway greenhouse effect may have been caused by the evaporation of the surface water and the rise of the levels of greenhouse gases that followed. Venus's atmosphere has therefore received a great deal of attention from those studying climate change on Earth.
There are no geologic forms on the planet to suggest the presence of water over the past billion years. However, there is no reason to suppose that Venus was an exception to the processes that formed Earth and gave it its water during its early history, possibly from the original rocks that formed the planet or later on from comets. The common view among research scientists is that water would have existed for about 600 million years on the surface before evaporating, though some such as David Grinspoon believe that up to 2 billion years could also be plausible.
The early Earth during the Hadean eon is believed by most scientists to have had a Venus-like atmosphere, with roughly 100 bar of CO2 and a surface temperature of 230 °C, and possibly even sulfuric acid clouds, until about 4.0 billion years ago, by which time plate tectonics were in full force and together with the early water oceans, removed the CO2 and sulfur from the atmosphere. Early Venus would thus most likely have had water oceans like the Earth, but any plate tectonics would have ended when Venus lost its oceans. Its surface is estimated to be about 500 million years old, so it would not be expected to show evidence of plate tectonics.
We can't rule out Venus at one point in time having an Earth like climate some billions of years ago. So we can speculate on the existence of a Venetian Marsh.
There are two types of Marshes, Fresh water and salt water, and we can immediately eliminate one of the two types on an attribute Venus lacks. Salt water marshes are formed due to the tide. Venus lacks a moon and as such would not have a defined lunar tide. I suspect we can confidently say if Venus did have salt water oceans, a salt water marsh would likely not exist due to the lack of a moon.
So issue number 1 - no moon.
That leaves us with fresh water marshes. On Earth, the https://en.wikipedia.org/wiki/Everglades Florida Everglades is probably the best example of a Fresh water marsh thats self sustaining. It is extremely dependent on Aquifers, but more over...Limestone formations. These formations allow the aquifers to exist and work to keep the PH relatively neutral. Limestone is primarily calcium carbonate, which is created by ancient sea life over the course of millions of years. Lets say for whatever reason, Venus was capable of supporting this ancient sea life and give birth to limestone.
This brings us to issue number 2...Venus lacks plate tectonics. On Earth, the plates shift over time and this limestone formation is relocated to the surface and becomes exposed land. There is no process on Venus that would bring this formation to the surface and it would simply be buried under more and more layers. Without this formation and the aquifers it brings, it's likely this marsh would never come to be. To go a step further...because it's lacking plate tectonics, Venus releases it's interior energy through volcanoes that includes a high sulfur content and would make the waters of a marsh harshly acidic without the limestone to counter that.
That being said...Fresh water marshes also come in the Prairie pothole variety https://en.wikipedia.org/wiki/Prairie_Pothole_Region It would require Venus to have a glaciation cycle, but may be the only example I can find (they lack drainage and fill yearly with snow melt). Life can contend with higher acidity levels (some life will even sequester the acids)...but the question of whether Venus could have glacial cycles needs to be asked. Not sure if I could find an answer to that one.
The other versions of Marshes tend to be seasonal, completely drying during the summer months.
Due to the lack of a moon and plate tectonics, I believe the marsh Venus scenario is highly unlikely...however with that being said, I only have examples on Earth to work with. On Venus, a Marsh-like feature that doesn't exist on Earth due to many other factors is still possible.