Croydon,
I have Lowrance HDS8's on board with a 1kw B260 airmar trandsucer, which is through hull fairing block, to be honest I wouldn't recomend buying a HDS5 for what you want to do, you will need at least a 7 and prefferably an 8. I run three on board, two on the helm and one networked and interfaced via ethernet to a tower. The image is sensational for the money, it's as good as you get in my opinion, the only time I have seen better is when people have spent tens of thousands on Navnet and the likes. To get any sort of definition between bait balls and fish in depths to and around 100FA you are going to need a 1kw transducer and a descent head unit. I have seen the bottom in up to 500FA on 50KH no problem and mark marlin on bait balls as well.
Just save for a little longer and get the HDS8, you will regret it if you don't.
Any thing else you want to know just ask.
Andrew