looking for advise, fishfinder transducer

I am rigging my SOT for fishing and need some advise on a fishfinder. Most I have seen include a transducer but they are meant to be installed on a transom. I have seen the videos that describe mounting the transducer on the inside of the hull. Do I need to buy a separate transducer or is there a specific FF I need. Any advise would be welcome. 

Depending upon the brand and upon the model, the transducer may or may not be included. You may or may not have the choice of transducers. So, you may have to spend extra for the one you want. Mounting inside the kayak requires a "shoot-thru-hull transducer, resulting in some signal loss but works fine for me.

You can use the standard transom mount transducer as a thru hull. It's probably the easiest since you just glue it down.
Only a few yaks like the Trident have a dedicated place for a transducer (molded scupper). So you can either do a shoot thru hull or a outside the hull mounting system.
On a thru the hull the thin kayak plastic probably does affect the signal strength but not that much.
If you are worried about signal loss or want your FF portable SteveS had a post on his tennis ball thru scupper removeable mount which would probably be less hassle than fabricating and rigging a transom mount.
When you are shopping make sure the package includes a transducer. Some "on sale" packages are a false discount because they don't include the $60-100 transducer.
The choice of FF mostly depends on your budget...

So, a transducer that is meant for a transom mount can be used as a "thru hull transducer" using Goop, Thanks, that helps a lot. I have read the tutorial and I think I am ready to buy.