Continuity mistake: After Dinky tells Big Mama and Boomer his idea, when Boomer flies over to Widow Tweed's house to knock on the front door, the water pump is beside the tree at the left side of the house, but when Widow Tweed opens the door and hurries over to the "pesky" birds, the water pump is now at the right side of the house. It continues to change its position in front of the house in following shots. (00:07:50)
Answer: Since we don't really know how he died, he could have drowned and not died on impact. Bear may not have been a strong swimmer or sustained too much injury from the gunshot to swim to the surface. However, in physics, force = mass * acceleration and thus larger creatures will hit the water with greater force than small creatures (as the old cliche goes; the bigger they are, the harder they fall). So it's possible for Bear to die on impact and not Tod because of his larger size.
Bishop73