Don't forget to develop the notebook!

d[d0_, a_, x_] := -(d0 + a x ^(1/2))

spreadPlot = Plot[d[2.3, 0.08, x], {x, 0, 1000}, AxesLabel->{"Distance from ridge axis (km)", "Water depth (km)"}];

spreadingData = {
{0, -2.3},
{300, -3.6},
{600, -4.4},
{900, -4.8}
};
spreadDataPlot = ListPlot[spreadingData, PlotStyle-> {PointSize[0.025], RGBColor[1,0,0]}];

Show[spreadPlot, spreadDataPlot];

explain = Show[Graphics[{PointSize[0.025], RGBColor[1,0,0], Point[{600, -2.5}]}],
Graphics[ Text[ "Average Water Depth", {880, -2.5}]]];

Show[spreadPlot, spreadDataPlot, explain];