I wanna get one of the Lowrance HDS 9 Gen 3 units.  Or a Humminbird Helix 9 SI for the boat we're looking at.  The Humminbird Helix MEGA units have pretty impressive sideimaging shots.  

They're all pretty darn , IMHO. 

Of course, those with good experience and expertise will probably be able to give you better answers and recommendations.  I just kinda threw out the ARFCOM "Glock 19!  1911!" answers.   What kind of fishing will you be doing and what features do you desire??  I'm still pretty amazed/confused at the massive amount of units out there as well.  

Also, I should have asked: how much are you wanting to spend?


ETA:

When I bought my first kayak, I went really cheap for my first fish finder.  I bought an inexpensive (~$100ish?) Lowrance X-4 Pro, IIRC.  It was very basic, black & white, with no real features other than 2D sonar.  I quickly got annoyed with it and wanted something with a little bit more capability and features, especially GPS.  I bought a Garmin Striker 4DV, which was about the same size, but was now color, has 2D sonar and Down-Vu (Garmin's name) imagine, and also has GPS that allows you to make waypoints, mark structure, and show you how to get back to your dock/ramp.  It really is a pretty nice little unit.  I want to say I paid a fair bit under $200 for it, but can't remember.

Since you have an actual boat, you have a lot more room to work with than I do on the kayak.  You'd really enjoy a larger unit/display versus these small ones I've had on my yak.  I'm really impressed with the Garmin units.  They are very easy to learn.  If you're not at all familiar with the capabilities of fish finders and trolling motors these days, watch some YouTube videos.  You can network certain units/brands together, and your FF can tell your trolling motor to follow contour lines, follow pre-programmed routes, etc.  It's really amazing.  So, to my original point in this post, you'll definitely need to figure out your budget, because you can literally spend from $100 to probably over $2000 on a fish finder.  Depends on your needs and how hardcore your fishing and/or desires for tacticool stuff is.

If you've never had a fish finder, they are frustrating to learn how to use, and understand, well.  Very frustrating.  I've spent hours on YT watching videos, so I'd say start there for sure!  

If you have a Cabelas, Bass Pro, etc near you, I'd head over there and just start playing around.  See which ones are easy to navigate with using the buttons/touchscreens.  I'm sure the Lowrance and Humminbird units don't take too long to get used to; I just felt like I was fumbling around with them for a while and got a little frustrated.  Of course, I didn't have a manual to reference either!  I took to the Garmin rather quick.

Now, I am by no means anywhere near an expert...I feel lost most of the time as well, lol.  I found a couple of Garmin units that aren't awfully expensive and should provide a nice setup.  One thing I've read, and maybe someone can back me up, is that Garmin's "LakeVu" map system may not be as detailed as other map/chart providers, such as Navionics.  That's all above my head.  Just something I think I remember reading during my research.

Garmin Striker 7SV

Garmin echoMAP 73sv

This Lowrance Elite-7 Ti is one that caught my eye.  I was thinking about it pretty heavily for whatever boat we'll get.  Keep in mind, to get certain views on various fish finders, you may need to buy better transducers (higher prices).  It's also capable of networking to other units/MinnKota motors.

Lowrance Elite 7-Ti

This Lowrance Hook-7 doesn't have side imagine, but is a decent 7" unit that has GPS, 2D sonar, and down imaging.  It also has a micro-SD slot that will let you put a map database in it ($$), if you wish...you can spend a bit more for one with the maps preloaded I think.  At $499, not a bad price.

Lowrance Hook 7

This Humminbird Helix 7 SI G2N seems like a nice unit as well.  The "SI" means "Side Imaging," and the G2N means it'll be able to network with other units/trolling motor.  You can buy most of these fish finders without Side Imaging/Side View, and will be a little less expensive.

Humminbird Helix 7 SI G2N

I listed all 7" units.  They seem to be reasonably sized for good viewing, and keep the price at a reasonable number as well.  You can go down to 5" or less units, or up to 9, 10, 12" units.  Some of the Lowrance HDS and Humminbird MEGA units have darn near photo-like imaging of the floor...it's amazing, you're gonna love it, I promise. /MAGA  That may be way more than you need, but it sure looks pretty!