SC above jogged my memory….
I had a hog do that, shot him next to a pond, he jumped in and sank like a stone. Sat down and had a snack until he bloated up to the surface.
I shot a Bull moose in BC decades ago…similar scenario….I saw him swimming across a lake, ran like the devil to get close to where he came out….shot him and he saw me as I released while he was shaking off. He jumped back in the frigid water and I quickly put another one in him right below the waterline.
Man, I think of that ice cold water pouring in…dang…he sunk right there…but was 1/2 floating in another hour.