Wouldn't get too bent out of shape around these 16mi, 24mi etc... radars on a sport boat. Assuming that you can get your radar 10ft off the water, and the ship you are tracking is 50ft above the water he will be below your horizon until he is at least 14mi from you or closer. That said, it will be difficult and nearly impossible to see boats that far out -- some exceptions but nothing I'd be relying upon for certainty.
If you have a need to track someone that far away -- get an AIS receiver -- much more reliable for vessels that are further away. I've got both the 3G radar and an AIS -- and the AIS is much better -- all commercial boats have them.
“Every ship, other than a fishing vessel, of 500 tons or more that is not engaged on an international voyage shall be fitted with an AIS, but if it was constructed before July 1, 2002 it need not be so fitted until July 1, 2008.”