Can a person safely jump out of a small plane going 70 mph and 60-75
feet high into a large lake without a parachute?  The stunt man has
experience falling off things and getting hit by cars, but this
particular stunt would be a first.  The pilot is well trained.  The
person will be wearing a wet suit, boots, maybe helmet.

Hi,

   Let's say the stunt actor falls from 75 feet at a horizontal speed
of 70 mph. By the time he arrives at the water he'll have a vertical
speed of 47.37 mph. But the composite velocity of this with the
horzontal speed will be 84.52 mph, and angle of incidence at the
water's surface will be about 39 degrees. So there'll be some tendency
to reflect off the surface, rather than cleanly slice right through.
It could be a rather complicated and risky endeavor.

He might be better off bouncing off the surface than "slicing clean
through".  Hitting the water at 84mph is the equivalent of jumping off
of a 240ft high bridge, which is something very few people would
survive.

Perhaps with some armor under the wetsuit the stunt man could survive.
I'm thinking of his landing on his back with plastic armor to protect
him there that holds his body stable  - stiffening his spine and neck
and holding his ribs in place.

But I am no expert.  Sounds like a good scene to use a dummy for.  
Qed100 can probably tell us how long the fall would last, probably not
much time for the stunt man to do anything of interest or importance
to the plot.  Flaying arms would be about all anyone in the situation
would do, which a dummy could also do.

Yeah. The fall from 75 feet will only last slightly more than 2 seconds. 

   forrest311, is there a compelling need for a living stunt actor to do the fall?

this is not something we can use a dummy for.  it's more about the
actual stunt than filming it.  The question is if it is safe.  Is
there a safer height the plane could be at or some sort of thing he
should wear...neck, back protection?  The bottom line is what kind of
injuries would you forsee?

With all due respect to the intelligence and broad knowledge of my
fellow Google Answer confreres, this is absolutely NOT the kind of
question you should be asking here!! Anyone even remotely considering
doing something like this should be engaging an experienced,
qualified, professional stunt team, and let THEM decide if the stunt
is feasible, and if so, how it should be done. This is absolutely no
place for amateurs, no matter how well-meaning.

No

If you really must do this stunt i would suggest 
1) a helmet. Water surface tension can be deadly at these speeds.
2) neck support. The last thing you want is your stuntman to be paralized in a lake.

No.  The stunt man can break his back when he hits the water.  There
is a maximum speed and height that helicopters should be moving when
they drop frogmen.  It's well, well below 70 ft and 60 mph.  Maybe an
experienced naval aviator can weigh in.  Don't do it.