That's a very common problem with many fish finders. Some are very sensitive to voltage drops.
The solution is to wire the fishfinder to the #2 battery and use #1 for starting. If you have an F250, you should purchase a Yamaha battery isolator/charger lead for about $70. This plugs into your engine's charging circuit and connects to the + terminal of your #2 battery. This will keep the #2 battery charged even when the battery switch is in the #1 position. In the event that the #1 battery runs down, you can switch to the #2 battery for starting. Otherwise, #2 is isolated from the starting system and no voltage drop will occur at the fishfinder when you start your engine.