It's difficult to seal around a wire and it's the most common leak on a cannister light. And if it leaks at either end (cannester or lighthead), the water can travel through the wire and flood the other end. Most multistrand wire has so many layers/parts, there's lots of tiny gaps that will compress when pressure is applied.
And check for a depth rating on that mic. Most 'underwater' mics I've seen are meant to be attached to a device above the surface of the water and dangled, not rated for several ATM.