We had the same problem when our boat was first launched. Builder
then put a small expansion tank in the pipework on the pressure side of the water pump and that fixed it instantly. The expansion tank is one of those with a rubber membrane separating the water and air sides so only rarely needs pumping up as no air is lost
to the water.
Although the pump was one of the type that normally doesn't need an expansion tank for smooth operation, addition of the tank improved the smoothness of its cold water performance as well.
Another cause as well as the expansion already mentioned is that if the hot water tank has a relief valve then leakage through that will cause the pump to cycle. Leakage may be due to the valve being faulty or else the water pump pressure setting being too high. In many boats this is easily seen as the relief just drains into the bilge
, but in some, like our own, the relief is returned to the fresh water tanks
and so invisible and may be missed.