Here is an onlinemapsources.xml entry for USA NOAA Nautical charts (Quilted). Some areas have tiles only at specific zoom levels so it's helpful to layer it (no transparency) over a base map.
<name>NOAA Charts Quilted</name>
<url><![CDATA[http://tileservice.charts.noaa.gov/tiles/50000_1/{$z}/{$x}/{$y}.png]]></url>
<minzoom>4</minzoom>
<maxzoom>16</maxzoom>
<projection>MERCATORESFERICA</projection>
<servers></servers>
<cacheable>1</cacheable>
<downloadable>1</downloadable>
</onlinemapsource>