The 4031 is anti-condensing, but I can't find a breakout board. I don't think it's necessary though, but I would need a regulator to step up the 3.3V.
What do you call the Sparkfun board then? That is a breakout board.
You can use the 5V pin of the Arduino UNO to power that board.
I'll be running this sensor in a very high humidity environment (90-100%).
And what temperature range?
Most consumer humidity sensors tend to have outages if the humidity stays at 100% (condensing) for some time. Sometimes the sensor has to be dried before used again. So in your very wet environment a sensor specialized for the environment might be a good idea.