water resistance problem, HELP!

[lalapsambel]

Here's my problem:
I have to find how much deceleration & how deep it will be when something fall into the water, but I just have the Vo when it strikes the water and of course Vt which is 0. I dunno how can I measure its deceleration underwater. OK, actually I just want to know how much force that the water gives when this thing strikes the water. So, I wanna combine the equation F.t= m(Vt-Vo). Can I use that? But I have to get either the time and I'll use Vt=0, or the Vt and I'll use the time = 0.01 s, so it's just instanteous velocity.

What equation should I use??? Please help me, it's a part of my project and if I can't solve this problem, it mean I should change the whole project![*(]

[HallsofIvy]

For any constant acceleration, you can treat "average" as (v0+v1)/2 - i.e. the average of the initial and final speeds. Here, v1= 0 so the average velocity is v0/2.

Decelerating at constant acceleration, -a, v(t)= v0- at so v(t)= 0 when v0- at= 0 or t= v0/a. The distance the object falls in that time is (v0/2)(v0/a)= v02/2a.

If you know the initial velocity v0 and depth d, then
d= v02/2a so a= v02/2d.