Garmin for gps, Furuno has made a good product for a long time. I always used a fish finder as the depth sounder, water temp, speed, etc. It may sound sort of dumb, but the last power boat that I owned had a GPS with an RS-232 (serial) interface, I bought the Navtech software and loaded it into the laptop and used the laptop for the chart plotter. It worked well for my needs and I hear the newer software has 3-D contouring of the ocean floor.
I learned RMS a little differently. Peak to Peak is 360 degrees of a sine wave if you were to o-scope the device. RMS stands for root means squared and is 180 degrees of peak to peak averaged over a square wave (rather than sine). The reason for this is that generally distortion begins to occur at or around RMS. The calc for figuring RMS to PtoP is to multiply (RMS) x 1.414 then multuply by 2 (for 360 degrees). To go from PtoP to RMS multiply (PtoP) x 0.707, then divide by 2 (180).
Also important is to ensure that you don't draw more power than what the alternator can put out or you will just drain off of your batteries. You should be able to get the amp output for the alternator. You probably won't draw too much with the above, but it's nice to make sure. Use Ohm's to figure out your load. (watts) / (voltage) = (amps)
_________________________
"Gentlemen, you can't fight in here, this is the War Room!"
President Merkin Muffley