My boyfriend got me a Garmin Striker 4cv fishfinder for Christmas. Neither of us has ever used a fishfinder, much less hooked one up to a kayak. After trying to figure this one out, the best we can tell is that it needs either an external marine battery or a "portable kit" that is listed on the box as an accessory but I don't actually see it for sale anywhere but Garmin, where it's quite expensive. The instructions also don't say anything about how or where to mount the transponder on the kayak - not even if it is to be in the water, kept dry, whatever. The whole unit is just feeling like it's for more experienced people who have used fishfinders before. Boyfriend said he wasn't really sure which one to get or what the differences were between 4/4cv/other letters. He said he also considered a Lowrance but it didn't get as good reviews as the Garmins, and  he thought the Garmin would be easier to use. I just want something that's waterproof, will tell me when I'm near some fish, will tell me the depth, and will let me record good fishing spots I find for future use. This was bought on Amazon and we can return it if we want to. Thoughts or advice appreciated.

I have the same FF.  I installed it on a rail. It is simple enough to use. It does all the basic things to find the fish and waypoints. I guess that's why no instructions.  I bought the transducer kit from Railblaza. They have it at Bass Pro.  Need to get a battery also. Connect red and black wires to battery. You will want to get connectors for the wires that match the battery connections. (I keep battery in plastic ammo box) Don't forget the battery charger. The other wires are not needed they are for sharing your data with other users.  I installed the transducer on the side and not through the scupper hole.

I have an older Garmin unit that I've been using for several years, and it's worked well for me. I tried all of the popular methods for mounting the transducer and settled on gooping it inside the hull---if you don't need to move the transducer between boats, I highly recommend this method, as it's completely hassle-free once installed. As mentioned, you also need a battery, and a 7AH SLA (sealed lead acid, which looks like a mini version of your car battery) is cheap and will be more than sufficient. There are lighter battery options, but they'll be more expensive and require a little more work to set up. You don't even need a box for an SLA battery.

Anyways, if you search, there's lots of info on here about mounting transducers, various battery options, etc. And, I think you'll find that fishfinder very useful and easy to use, so I'd recommend that you give it a try.

Thanks for the responses.  I will have a small lead acid battery.  Will wire it to supply power for emergency lights and possibly charger for camera.  Didn't know you don't need a dry box. 

I'm planning to mount the transducer through the hull using the swim noodle approach and see how that works.  If it works it's the cleanest.  Maybe even try to silicon caulk permanently.  But I don't know how much signal or clarity I'll be missing comparing to mounting directly in the water.  I understand the Garmin Striker 4 transmitter is only 200W (instead of 500W) so signal attenuation through the hull maybe a big factor.

Will see.