Then don't let them do it. If they don't have a clue, you don't want to be their guinea pig on something they know nothing about. Adding an external antenna is not a big deal. There are many shops in Texas that can do it. Ask here for the part number, order your own, have someone install it.

As to the accuracy, the navigation system (irregardless of your boat and it's equipment) has limitations. I believe the GPS system is hovering around +/- 3 meters or a little more now. That's pushing the quoted +/- 10 feet some refer to, and that's probably not far off. While you might go to a known spot one day and get right on it, atmospheric density and frequency drift can change, and you'll get put on a slightly different spot 3 hours later, or 3 days later, for instance. It's about consistent repeatability. Consider you get that 10' accuracy when plotting a trip from here to halfway around the globe. That's astounding, actually. That system drift (uncertainty) is still as much as 10' when you move the two points much closer together. On many days you may see it better than that, but if you have a transducer at the stern, and the internal antenna on the display at the console, you're adding a few feet of "offset" and to a fisherman, that will be labeled as "error" because it puts him/her off the desired spot.

I guess it’s not a big deal when bass fishing to know I’m close and can cast around and near. Just trying to figure out the best way to consistently put myself on top of structure for crappie fishing straight down.

Probably carry some buoys. And get some on the water instruction.

Oh and yes I told that dealer not to worry about it.