Whats wrong with my jeep

Hi, so my jeep would start. I thought it was the
battery, so i checked it by puting in a battery  that i
knew it worked.. turns out it wouldnt turn over, i
figure it might be the starter.. so i hit on it a couple
times till it did turn over but that was it..due to my
budget i decided  that instead of buying a new one,
i would take apart the old one and fix it.. so after i
abought the parts and put it all back in place still
the engine would not start.. i then pull out the dip
stick for the engine oil and it looked like there was
water mix with it... what i would like to know is how
the water got in my oil?
If any of you could help me answer this question, i
would really appriciate it. . Thanks