Your correct. My phone can show me the mph i'm running anywhere but our phones ain't designed to get the crap beat out of them. I've use Garmin and never had a issue is why I like them. They are built tough and on THT > The Hull Truth . com my offshore boating forum god help a unit if it does not stand up . Those boys will hang it by the nearest sail boat mask. LOL
Car Gps, road stuff and our phones might last a while but the first hard bump, you know that deal.
$510 cheapest I found on e bay new. Used on there now for $450 used. Mount is about 50.
eBay item number:252446767933

Will a cell phone work, yes. Everything depends on how you feel about it when your 30 miles off the beaten path. 65 miles out to the stream. I ran two garmina in the Fountain. One for sonar one for GPS both units had gps if one went down.

 

i vote for gps over phone and use phone for backup