99 gmc 1500

I have a 99 gmc 1500 5.7 bought the truck guy said
it needed fuel pump came wit a new one put it in
ran fine hurricane came threw and got water in the
tank got it all all put a full tune up on it then n the
last couple weeks after sitting for awhile won't
start put a dash of gas n throttle body fires up idles
fine but has no power won't even spend tires on
dirt every once n awhile it will fire up fine now I
have to put gas n throttle body to fire up every time
I drive it if I ease on the throttle it goes down the
road but fills like it's loosing something get down
on it fills like missing n tranny slippn but tranny is
fine need help asap