In the northern hemisphere only, what percentage of the surface is land?

From http://www.physicalgeography.net/fundamentals/8o.html and http://www.eoearth.org/view/article/154984/

The spatial distribution of ocean regions and continents is unevenly arranged across the Earth's surface. In the Northern Hemisphere, the ratio of land to ocean is about 1 to 1.5. The ratio of land to ocean in the Southern Hemisphere is 1 to 4. This greater abundance of ocean surface has some fascinating effects on the environment of the southern half of our planet. For example, climate of Southern Hemisphere locations is often more moderate when compared to similar places in the Northern Hemisphere. This fact is primarily due to the presence of large amounts of heat energy stored in the oceans.

This equates to a measurement of ~40% land in the northern hemisphere. This agrees with the northern hemisphere azimuthal equal-area map image below.

The surface area of the Earth covered by lakes and rivers is a fraction of a percent, so their classification does not really change the answer. However, you would get a slightly different answer if you consider land to be any continental crust (including seas not on ocean crust). If you define a year and a definition of "land" that you are satisfied with, an exact calculation could be made with GIS software.

From https://www.fba.org.uk/journals/index.php/IW/article/viewFile/502/327

Rivers and streams are likely to cover 0.30–0.56% of the land surface [of Earth]