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
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