That's correct...the V is rated waterproof but real world experiments say otherwise. Having a III+ model (same form factor as the V) on the boom for many seasons, it only got heavy spray which is handled fine. A capsize to turtle ended it's life. Garmin has a flat rate re-furb fee of $125. Ebay had the V for $180 as a refurb. Insurance was covering it so went with the V.

Don't have any photos of the mount handy. But, I used a standard Garmin handle-bar bracket that clips & screws into the unit. I drilled a hole through the wedge-shaped section of the bracket to accept a quick-pin. Then I made up an articulated bracket out of a few bits of stainless flat. One end of this fits over a vertical screw I've mounted to the top of the boom. A wingnut holds the bracket down and allows horizontal rotation. The gps end forms a fork over the wedge section of the garmin bracket. The quick pin passes through the forks with the wedge inside.

From the trap (gunwale is ~5' off centerline on the T) I can easily glance at the gps for a heading (typically I display the big compass pointer when distance racing) and speed (or distance to mark etc) reading without losing concentration on my helming duties.