You will be able to, but it depends on what brand depthfinder you get. I am pretty sure most Eagle/Lowrance models come with a mount that can be mounted either way, depending on what you need. I'm not really positive about the others.
I just bought an adapter for my ff to mount on my trolling motor .its basically just a clamp for a dryer hose,with a piece of plastic at the end.cost 9 bucks i think.
Yeah, Also, like pitdog mentioned, If the depthfinder you get happens to not come with a t/m tranducer mount, you can always buy one. All of the manufacturers will have a t/m mount.
Most Eagles come with a transom mount. I bought two different models about this time last year and that's what both of them had. I had to buy one of those $9 hose clamps for the bow mount.