My original logbook was stolen from car last year, so I knocked up a quick spreadsheet which just records the date, location, depth, time, gas mix and water temp.
If I'm teaching or DMing OW courses, my little rule is : If I get out of the water between dives then it is 2 dives otherwise just one. And, I must confess, I rarely log boat scrubbing dives, even if I've spent an hour plus u/w.