Humminbird sidescan. I would get the 99X model with Navionics loaded if you have the money. The larger 1100 series is still a bit buggy I understand. I would also get the external antenna model.
I dive from a 19 footer also, in this case my BW Nantucket. I find a 19 foot boat quite a good compromise for diving and long distance towing. If I did not have to tow I would go bigger. That said, you should have room in your console to accommodate the Humminbird and if not look at their smaller 79X series side scan sonars.
Some people think the side scan is a joke or gimmick but they are wrong, it does not always work, transom mounted transducers are an issue since they move with the boat vs a tow fish--- BUT--- much of the time you will get decent results and see things you did not know were there. N