When you're drowning, each 'tick' (about every second) takes ten percent of your current maximum hit points, but it 'rounds down' to the nearest 10 hit points to do that calculation. I tried creating several different starting parties and what I found was that if a character had 20 max hit points, he would lose 2 on every tick. A character with 28 hit points would also lose 2 on every tick. I was never able to create a character with x9 hit points (39 for example), but it clearly rounds down from 38 to 30 or 28 to 20 to do the math. Bottom line, if you have fewer than 10 hit points (or at least 8 or fewer), it rounds down to zero for the calculation and you never lose any.
