It can be confusing even with the user manual. I had the same problem but figured it out messing around with it on the water. Anyway, as the previous post mentioned make sure the transducer you are using supports sea temp. Then do the following:
1. Press the MENU button
2. Select "Databar setup"
3. Highlight the configure option but do not press any button after doing so.
4. Now, this is where its tricky, look at the bottom left of the screen. You will see an option that will say Add Data. Press the left most button on the bottom of the screen (the one below where it says add data).
5. Another menu will pop up. You can then select environment and then the sea temp option. On this same menu you can add wind, swell, etc. if you have the applicable sensors.
Again the key are steps 3 and 4, then it will be straight forward on the screen.
Just a quick note...I have a C80 (I use it to view my fish finder, GPS, Chart plotter and Radar) and it took me at least 2 hours to program/customize all the different screens to get it to where I wanted. I would recomend getting out the manual and playing around with it for an afternoon until you have all the screens customized the way you want. Then go fish and modify it again accordingly.
Exactly what H.N.F.C said...mine is a C80 as well...I've had it for a while now, read manual several times, but every time I go out and play around with it I learn something new....lots of great things you can do with it...get the navionics card for it...well worth it...
Are there any good threads here about using the fishfinder on the C80. I just got one a couple trips ago, and haven't had much luck using it. Seems like even when I am catching fish, nothing is on the screen. It does work, because it shows bottom depth and sea temp.
Your C70 should have come with a plastic laminated card (2 sided 8 1/2 x 11) that has quick set-up and basic settings, etc. ... if it wasn't in the documentation, PM me and I'll scan it for you. RayMarine has pretty good info. package available online on it's website too.