I don't think you're going to gain much over the P66 you are already running. Sensitivity should be about the same, with the upside that you might get a bit better target separation. If you want to see a noticeable improvement over your P66, you might want to consider going to a SS175HW (i.e. 1 kw).
For salmon / tuna, the HW versions of the transducers are the most popular and effective as they cover the most water. However, I don't think Airmar makes a SS75HW unless it is very new. They make the standard SS75 (narrow beam width) and the B75HW (bronze housing) which would work if you have a fibreglass boat.