Sand the area of the cracks and apply a patch of fiberglass cloth and fiberglass or epoxy resin mixed according to directions. Overlap the crack by at least an inch on each side of the crack. Let cure and sand again and apply a finish coat of the same resin. If you don't have resin and cloth on hand, you can buy a kit from NAPA with a small quantity of each. Because this area gets a lot of flexing as you stand in the cockpit, you can't depend on a mending putty like MarineTex. It's an easy repair. Good luck.
Real cracks should be done like Alan said.
Fine spider cracks that only effect the gel coat are not worth chasing in the traditional ways. I tape a ring around the inside of the tub around 2 inches up from the bottom. Then I lightly scuff the bottom with 320 sandpaper an Scotch brite pads. Spray the bottom with white Flex Seal spray then peel the tape around 12 minutes after spraying. This gives a durable non-slip surface that seals the tub.