Fish Finders?????

[Poncho]

I have been kayak fishing for a few years now, and in the past I have tried to split my time fishing equally between kayak, surf, and bumming rides offshore with my friends who have boats. But for some reason this year my but has been glued to the kayak, I just cant get enough. I have been tossing around the idea of getting a fish finder but I am just not educated enough to go out buy one and install it, so I am reaching out to the yak rats for help. Just a few questions.
1. Are they really necessary?
2. Do I have to drill a whole for the transducer?
3. How heavy are the batteries?
4. Cost? Is it you get what you pay for, or are the less expensive models just as effective?
5. Durability to water damage?
6. Last but not least, What are the options for mounting fish finder to boat.

Thanks you guys in advance, any help is greatly appreciated.