Lead acid battery boiling during charging: this is bad, right?
Yes it is normal for the water to boil if you are overcharging it, with a too high current. And it is bad.
Yes your charging current is set too high. Also check that the final float voltage from your auto charger is correct to the final charging voltage of your battery. 


From Yuasa batteries (pdf): yuasa techmanual


  For the correct charge rate a rule of thumb is to divide the battery’s amp hour rating by 10. For example a 14 AH battery should be charged at 1.4 amps (14AH÷ 10
   = 1.4 amps).  See  the  section  on  “Choosing  a  Battery  Charger”  for
  more details. 
  
  When charging amperage exceeds the level of the natural absorption rate, the   battery may overheat, causing the electrolyte solution to bubble creating flammable hydrogen gas. Hydrogen gas, when combined with oxygen from the 
  air, is highly explosive and can easily be ignited by a spark.  



Yes.
You shouldn't boil them. But some evaporation will still occur. Even "sealed" batteries have a safety valve for venting if they are overcharged.