Although the 18 Volt  battery may be at 21 Volts unloaded as soon as a load is applied within seconds it will probably drop 1 to 1.5 volts.  I doubt that would damage a detector does it do you any good to have that higher voltage.  I ran my detector on an adjustable powersupply to and measured the current  and I even found my old data.
Eagle fish Mark 320 current draw. Voltage is
power supply meter with a fluke multimeter
in series to measure the current. Transducer is
glued to bottom of canoe not in the water.
Volts----Imin----Imax------Iave
8--------84ma----332ma--104ma
9--------88ma----372ma--111ma      
10-------88ma----456ma--116ma      
11-------92ma----476ma--121ma
12-------92ma----476ma--131ma
13-------96ma----612ma--132ma
14-------96ma----448ma--136ma
15-------100ma---492ma-141ma
16-------100ma---532ma      -148ma
17-------104ma---560ma      -153ma

As the voltage goes the current also goes although not by the same ratio.  If the detector performance also went up it might be worth it.  I do not think there is any improvement in detector performance above 11 volts.  My detector works well at 10 volts.
The 4 Ah 18 volt battery is probably made out of five 4 Ah 3.6 volt batteries in series.  If you run my detector on this battery it would start off drawing more than 153 mA and when the battery was drained it would be drawing about 140 mA.
If you cut that 4 Ah 18 volt battery apart took 2 of the cells and cut them off and threw them away the battery would be lighter and run the detector for longer.  The voltage would be between 10 and 12.5 Volts which would have a current draw of about 120 mA. To see the voltage profile of a 3 cell LiPo battery see the second graph on the page.  It is labeled LiPo5800.