Question about batteries and depth finder

[MississippiBoy]

My birthday is coming up on January 6 (if anybody wants to send me a present, let me know and I'll PM you my address ), and the wife has decided to give me a depth finder for my little boat. Nothing fancy, maybe a $200 or so unit, but that beats not having anything at all, right?
The boat only has one battery, but I have heard that it's not a good idea to have the trolling motor and the depth/fish finder hooked to the same battery because of interference issues. I think I have room under the front deck to put this big battery to dedicate it to the trolling motor. That means I'm looking to get another battery to use as the cranking battery and to run the fish finder.
My question is, how big of a battery do I need for the 40 hp Yamaha, depth finder, and bilge pump? I'm thinking fairly small, to keep down the weight, but I'm not sure how much current the fish finder will draw.
Any advice is appreciated.