Underwater bondage
From wipipedia.org
Underwater bondage is being in bondage while floating underwater.
The victim is weightless, so feels free and easy. However, the victim is also stringently bound so cannot move and feels utterly helpless and vulnerable. The interaction of those two contradictory feelings creates a unique sensation.
Warning: There is a serious risk of drowning in this activity, so the victim must be closely monitored at all times.
