Can I splice off my console fish finder power wire for the bow fish finder?

It's a hds 8 at console and a 5 at bow.

Same fuse size?

I'd run straight to the battery. Depending how long your boat is, you may need bigger wire to have 12v to the front. Last time I wired mine, I used the same wire size and up front only showed 11.5v.

No. I ran into that issue. Had to run a direct line for unit to perform.

I guess I'm lucky. I have a switch from my main panel that feeds a sub-panel for all of the electronics. The sub panel has fused outputs for VHF, 2 fishfinders, stereo, and the Humminbird ethernet hub. Everything up to the subpanel is fed with large wire and I have no problems with any of it. The biggest problems with electronics are caused by either dirty connections and wire used that is too small and causing a voltage drop. If the wire is sized correctly and all connections are clean, you should not have issues with low voltage.