Yamaha's got an app....I "think" it also works with the new F25, but I could be wrong. Worth researching.
Yes I use my phone app, I just use maxspeed app. Last year in August I went up to the bigger lake for some crappie. Got there before day light, waited a while so that there was enough light to navigate. I was going about 3 miles to my "spot", which requires going across open lake for about 3/4 mile, making a hard left curve down a channel, then open lake, then to an island. I fish near the island. There was plenty of light to see, so I left the ramp and headed out. Got around the bend and it hit me...a fog bank like I've NEVER seen in my life, it was so thick that I couldnt' see the front of the boat (and it's just a 15' tin). I had the app turned on of course. I knew "about" where I was but with the fog that thick it was hard to tell where the banks were, the rock pile, and submerged trees. I've run that route many times; and had it plotted in the app from the last time I was out. So I zoomed all the way in and stayed more or less on the line until I hit the waypoint, stopped, trolled about 5 foot and found my tree top. Saved my tail end that day.
THe MPH isn't all that accurate, but close enough to give you an idea. It's not "real" GPS, but as said, close enough to give you an idea. Mine puts me within a 16' radius at the closest point which where I'm at is close enough. If I were on the river, 16' is the difference between running into a nasty pile of rocks and squeaking past with inches to spare, so I don't use it much there. If it's foggy there (which it is almost every day), I just use my gut...and stay real conservative.