I have a small boat that I use my 24V Vantage on and was wonder if I can hook up a 12V fishfinder some how or do I need a seperate 12V supply? TIA

yes you can. Just hook-up your fishfinder to only (1) battery, as shown below. Keep in mind though, you may experience "noise" on the screen, while the Vantage is running. If so, you may want to think about using a small, lawn tractor-type 12V battery for the FF.
HRG