gps speedo; need help

[76 Bonneville]

I'm trying to hook up a gps speedometer and the instructions describe both "battery ground" and "vessel ground." The battery ground is obvious but what do I do with the vessel ground? Can I just solder it in with the battery ground or does it need to connected to something in water?

[VAMI]

Is it a Gaffrig? I hooked mine up a month ago kinda forgot. sender , gauge hot and ground then briaded ground.I soldered to the braid and grounded it on the boat.

[SmokinLowriderSS]

Everything eventually grounds to the battery (except non-conductive fibreglass boat hulls), or it isn't grounded, and does not work.
My Nordskog digital works great. :D

[H20MOFO]

You Probably All Ready Got It But I Think He Is Saying Take The Other Ground To The Block, Cyl.head, Ect. BY THE WAY WHILE WERE ON GPS'S DO THE BOAT MOUNTED UNITS UPDATE QUICKER THAN A HAND HELD?