A level of 352 is a bit high, but, as your doctor suggests, probably just needs watching.
It needs to be considered with iron levels, and total iron binding capacity.
Ferritin is an acute phase protein which means the levels we measure are elevated for a variety of reasons - most of them quite harmless.
The significance of high ferritin (as a disease marker) relates to the iron overload disease Haemochromatosis. There is a simple genetic test for this which rules this out.
The treatment for patients with haemochromatosis is simple - we just take a little bit of blood from time to time forcing the body to use up the extra iron to make new red cells.