Render artifacts and general lighting trouble

Hi All,

Having some issues getting an attractive image. Must confess that I have very little experience with rendering so this is all new to me.

As you can see we're getting some artifacts on the water... They don't look as harsh here with the size reduced... Attached full size.

Also struggling to get the sun dim enough for a night shot. With channel 0 (sun) set to 0.0001 it's still too bright and at 0.0000 the ambient light is completely gone. Obviously I'm missing something.

Ryan