[CharlzO]

For the first, yes, it's using the GPS to calculate your speed. However, if you want to add things like water temperature, and have a more exact speed constantly (some plotters only update every second or two, which is fine for some people. Other people like knowing every millisecond), then you would need something like you linked.

Many people feel that a thru-hull transducer is more accurate. However, depending on just how vital an exact accuracy and over 200' deep goes, could make a case either way. Some seem to be less accurate past 200' from what I've read, but it likely depends on the unit and if it is installed without air in the medium, etc. I use a shoot-through-the-hull style (like this) with my chartplotter, and it's just as accurate as other people around me on the lake, and will easily read the 450'+ depths in the middle of the lake. And I didn't have to have any extra holes drilled

 

[CharlzO]

Correct, they won't give you water speed or temperature, just depth (and fishfinding). In my case, I was happy with the GPS speed instead, and if I want to know if the water is cold, I'll stick my toe in lol. That, and the cost of the multi-function thru-hull was a little more than I wanted to put into something I was already putting money into. But it depends on each persons location, sailing habits, etc.

I do believe you're correct that it would be unit-dependent. I never looked into it, since i went the route I did. I'm sure Garmin can tell you, or even perhaps download the owners manual / installation for that unit from the web and see if it mentions setting the inputs based on what transducer, and so forth.