If I might chime in, I think that if the air flows out of the dryer vent and then bubbles out of the water, you will have created a humidifier. I doubt the heat value will be much.
OTOH, Edison was ridiculed about his ideas for a light bulb, but he kept notes on what did and didn't work and eventually he succeeded. That's what I love about these forums, we try to prove whether an idea works or not and then move on.
So, give it a try, keep good notes and good luck with your idea! Track your heat output and the humidity levels so you can share the results with the rest of us. {I looked and could not find where you are from, so I don't know if humidity is an issue for your area. It really is an issue in my area.}