To answer the OP, there is no (electrical) fan disengagement like some other vehicles have. While the HMMWV is a well engineered and capable off road vehicle, it is not meant for "muddin". I'm sure the operators manual describes a well-thought out entry with spotters (soldier B & C) as well as pre-entry depth checks and the like. YMMV. Proceed at your own risk.
Not made for muddin? Is any vehicle for that matter? But I disagree...ive only ever seen one broken fan blade, and that was due to a misaligned fan shroud. Which I suspect may be the OP's root cause. But I wasn't there to see what really happened. Can't tell you if the fan was locked or in freewheel. Those fan blades are freaking incredibly hard to break.
you won't just snap a blade off...beleive me.
I had a 95 H1 that when I went off-road and the body started flexing due to extreme angles, the fan blade would strike the
shroud...it was mis adjusted.
There is a metal clip as well that goes from the radiator to the top of the fan shroud that keeps the shroud away from the blade During flexing. Is yours present?