Yes I'm afraid that is intentional. The serpent meat always used to float, whilst the scales sank to the bottom (even prior to the update) and yes they do expect you to drag the serpent on to land using the harpoon. Admitedly a bit of a hassle to do in singleplayer, but possible with preperation (as in having a high stamina bar whilst slowly dragging the serpent behind you, to make sure you don't deplete said stamina until the last bit at the shore). The Frostner usually does short work with serpents (one or two hits), but that is pretty much considered an endgame/late midgame weapon. The shallow pool suggestion mentioned above sounds good though.