Accoring to Humminbird's product website,
for side-imaging with the 798, it looks like you only have
two choices for transom mount SI transducers:
XHS 9 HDSI 180 T or XNT 9 SI 180 T.
The first one is physically longer, so it will probably give
you better images than the compact one, but you can't use the 800khz
capabilities of it.
It is also more expensive.
I'd let Greg Chime in before I made a purchase.