I tend to bath my BP a day or two after they have had a really messy meal.
We like to handle him and let him roam around while we clean his tank and water dish.
I guess it is really up to how they are feeling cause there are times that he freely dunks his head while moving in and out of my hands and there are times that he does seem to "swim underwater" with his head under for longer than a dunk. Admittedly, I don't think that it is more than 10 seconds but it doesn't seem to harm them.
I never force him to keep his head underwater...it's all him.
If I want to "force" him to soak, I just make sure that there is only enough water to cover half his body and stick his hide in the water. He just goes in there and coils up with his head at the top.
Here's a picture of him soaking outside. I had his hide on top of his water bowl and he stayed put (hide taken off for picture).
Bit more water here cause it was a warm summer day.
DSC_0140 by closhusan, on Flickr
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