I will second what luenthewind said.
I have the Lowrance Elite 7 TI with tripleshot transducer. I have it just in front of my kayak seat. Good size for sonar, downscan and side scan up to about 30'.
Beyond that need to enlarge the image to see things well on side scan. I have missed several things on side scan out wider that I have caught when replay the logs at home.
Figure in how good your eyesight is and where you will mount the finder into your equation.
As for corrosion, I have about 170 outings on my unit in fresh water with no issues. I take it on/off the kayak each outing.
I use the
Nocqua Pro 10AH Battery.. It is about the size of your fist and I have run my finder for over seven hours in daylight(maximum brightness) on a charge.
Link did not show up, I use the Nocqua 10AH Lithium Ion battery.
You did not mention anything about ability to save waypoints or showing maps.
If you go this route, which is very useful, and want to save some money, think about genesismaps.com for your maps. Get the basic maps that come with unit then get the maps for lakes you are interested in free from genesismaps.com.
It is a social mapping site, you can send your logs in and they will update their maps and make them available in a couple of days. You do not even have to send in any logs to get maps.
They do not cover the whole lake but cover those areas where kayakers frequently are which is close to the ramps and put in locations. You can go see what they have for where you are.
If your finder allows saving logs in an outing or 2 you can send in enough information to have their maps reflect an area you like that is not currently covered.
A finder will make a big difference in your fishing.
Best of luck which ever direction you go.