Typically, temps in the mid- to upper-60s. March, April, and October are the best months to visit in my opinion. The crowds are reasonable, restaurants are uncrowded, and weather is comfortable.
We've been to HHI twice in early March. One time it was very pleasant, in the mid-70s and sunny almost all week. When we went again, it was mid-50s, cloudy, windy, rained most of the week. We enjoyed it both times.
Have been once during March, actually during the St. Patrick's day week. Weather never got above 50 and rained the entire 5 days I was there. I know others have had nice weather, so you just never know.