We went thru the same thing - it is quite unpleasant but it also was yet another reminder of how much I clearly love this boy of mine! Who else's poo would I scrub off the tub while simultaneously thinking "well good, he has a good, healthy digestive system." :)
We did a few things. I did not want to change our "bedtime routine" so we moved dinner a bit earlier to give him a little more play time after dinner and before the bath. And for a while, we just had to keep the baths shorter than usual. We watched his little face for any signs of "activity" about to happen and would scoop him out if we saw any such signs. We also kept the water even more shallow than usual and less warm than usual...it's not cold but not overly warm either.
Ultimately what changed was his appetite, I think. They say that they eat less after the first year...we saw his appetite decrease around 14-15 months. He is a good eater, but he just doesn't eat as much at every meal as he used to. According to research, this is normal and is because they are not going to triple or quadruple in size any more, like they did the first year, so they don't need as much food. I think his lower intake of food has made the most difference.
So, there are things you can do, but this phase will pass (no pun intended).
Good luck & God bless-
B.