It could be due to a number of factors I guess... I'm only speculating here by the way....
<ol>
<li>Wind is usually more gusty inland and hence not as consistant power...
<li>Most likely one though is that fresh water is less dense than salt water and hence you need more power to get planing.
</ol>
Must admit I've never kited in a lake or river due to gusty winds etc...
A also find an interesting thing when kiting... in a cold dry wind (like we get in Auckland in a Southerly during the winter) the power is much more than if we get hot muggy stuff from the NE in the summer despite the same wind speed...
It's called air density and it has the same effect as kite size
<span style = "color:'ff0000';font-size:20pt">BLOWN AWAY</span>