It would not give longer life to have 12 of those in series although it would do no damage (12 batteries 1.4 Volts each = 16.8 Volts).  You could label the batteries 1 through 12.  For the first day of fishing use batteries 1 through 10.  That night take 1 & 2 out and replace them with 11 & 12 for the next day.  That night remove battery 3 & 4 and replace them with 1 & 2. Continue with this sequence and after 6 days of detector use your batteries would have 5 days use on them.
For this to work you would need to use the detector less than 3.8 hours per day.