I drive three or four of 1/4 inch holes through them. The holes will damage the platters and the resulting metal pieces will contaminate the insides enough so the data will not be recoverable... add in any dust that gets in there, too, plus the electronics which control the drive will be useless. Add a little water to start the insides oxidizing if you are really paranoid.
Why? In the hard drive, there are one or more round metal platters coated with iron oxide - similar to the stuff on magnetic tape. To read and write data, an armature moves data head across the platters looking for the appropriate data stored in "sectors". When the case is exposed (because of the drilled holes), dust particles will be attracted to the platter surface. The metal shards from drilling will also stick to the platter surface.
As an engineer friend used to describe it: The platters spin anywhere from 5400 to 10,000 rotations per minute depending on the hard drive. The armature flies the heads across the surface of the platters to read/write data. The heads cannot touch the surface of the platter - and are VERY close to the platter surface. When all this is going on, if there is a sudden jolt and the head does touch the platter surface, that head crash can scruff a very small area of the platter rendering it useless.
The analogy: Imagine a supersonic jet flying over a glass-smooth surface at mach 2 (2x the speed of sound) at an altitude of 2 feet... and hits a 3 foot tall rock.
Yes, dropping the hard drive in water will eventually ruin the drive - if you go this route, get a screw-driver set and open the drive case, first. Dropping batteries won't provide immediate help - they will need to leak their contents to do real damage and the result would technically require hazmat response.