As much as level 9 parties might fear this thing, its 20 RHD kind of ruin it for player use.
Let's take a closer look: undead, medium-sized, 25 strength and all other stats average, two 1d8 slams (but drowned can wield weapons so nobody cares), unholy toughness, fast healing 5, and a swim speed. None of these are bad traits, they're just spread quite thin.
The drowned's main attack is their Drowning Aura, which is admittedly pretty neat. It's free to use, affects everything that needs to breathe something other than water, and is resisted with a constitution check rather than a saving throw.
However, at ECL 20 the majority of monsters is either immune to drowning or in possession of heaps of constitution, making it quite hard to effectively use this ability. What's more: a drowned's allies aren't immune, and will probably have constitution scores below 30, causing them to die very easily if they get too close.
Considering this all, -0 is the only LA I could assign here. I'm sure nobody will disagree.