I’ve been using the Garmin 78scx for about the last 7 years. When in use, it’s in a clear drybag attached to my sprayskirt.
I keep the GPS in the DryPak bag along with spare batteries and a grease pencil for making notes on the deck. To absorb humidity, I put a small handful of rice (about the size of a small lemon) packed into an old stocking. Squeezing the air out of the DryPak (before closing) helps to minimize the buildup of humidity on the clear vinyl window. Despite all these efforts, some moisture will always appear inside the clear bag.
Using a hot glue gun, I added a few 1/2” webbing loops to the front deck of the sprayskirt. Attach the DryPak with keychain carabiners for daytrips. Be sure to have the drybag opening facing aft and not forward- in case you get out of the boat with the DryPak open, the GPS will not fall out. For multi-day trips, I sometimes use plastic zip ties instead of carabiners. Be aware, it is NOT ideal to have these added loops and little carabiners on the sprayskirt as they could be a possible point of entanglement while getting in or out of the boat.
The hot glue has never failed and generally will outlast the skirt. The DryPack lasts about two seasons, but as yet, I haven’t found one of better quality that also has the attachment points.
To improve the waterproof seals on the GPS battery compartment, I lightly coat the rubber gasket with o-ring grease I get from the dive shop or a plumbing supply store.
It’s not a perfect system, but it does work fairly well. Sunglasses with the “reader” lens along the bottom help to read the tiny screen on the Garmin.
Here’s a link to the DryPak:
DRY PAK Multi-Purpose Case, Clear, 6 x 9