There are more than one "falls". There is Trafalgar Falls, the Emerald Pool and Titou Gorge, which are the most popular destinations on Dominica. From your description, I'm assuming you are referring to Titou Gorge. When we were there, there were quite a few people in the water, and this was midday, around noon. (We had already done a hike to Middleham Falls,which is a more strenuous hike). There is a larger area where you enter the water and swim a bit before the current pulls you into the gorge and that's where the falls are. It's very pretty and can hold quite a few people. Since the water is fed by the waterfalls and it is also in an area with lots of overgrown vegetation, the water is rather chilly, so not too many people stay in long, so I don't think you have to worry about crowds in the water at Titou Gorge.
Now, at the Emerald Pool, it's different. It's a hike down to the pool and the pool is smaller, so if there are crowds, you may have to wait a bit for a turn in the pool, but it is really a lovely spot there.
Have a wonderful time in Dominica!