Getting ready to install new unit and lowrance manual says to use their epoxy to place transducer in hull. Of course no epoxy came with the unit.  Can't I use any kind of marine epoxy for this?

You have to use a slow cure Epoxy and mix it slowly to avoid bubbles. After you mix the two parts, put a glob on the face of the transducer and press it in place in a twisting motion to get most of the epoxy from between the hull and transducer. You want the boat hull to vibrate the same amount as the transducer face without some of the vibration being absorbed by the epoxy or air/chemical bubbles in the epoxy.

the last one i put in i used devcon the  30 min type  it cures a little slower   us a wooden stick to mix the two part epoxy  like a ice cream stick and do the thing   wayne said  then i put a weight on top of transducer  so it dont move. till good and dry

If you want the good stuff, order some of this:

http://vexilar.com/pages/accessories/accy_transducers_ace.html