I'm going to the Big Island in May and plan on doing lots of snorkling and scuba diving with my wife. Do I need a wetsuit for both activities?

For scuba, definitely.

For snorkeling -- only if you have zero body fat to keep you warm (never an issue with me...I'm well-insulated!)

Gawd, I love you Auntie M. What would this site do without you?
8-)

I was in Hawaii mid-March a couple of years ago. The weather was warm enough to wear shorts and bathing suits but I was always cold in the water. Personally I wish I had just a wet suit top on. One time it got cloudy while we were snorkeling and I came out of the water shivering. But the fact that you will be there 2 months later than I was may make a difference. Have a great time, I think about going back all the time.

I've found that a tee shirt is enough for extended snorkel jaunts (plus it's good for sun protection). My daughter likes her short-sleeved poly ski underwear. Scuba--wet suit for sure.

The watertemp in Hawaii is always surprisingly cool. Everytime I enter the water and hit that certain part of my anatomy (armpits!) I am a little shocked.
I am a big guy, but can only snorkel for forty minutes or so without becoming chilled. I get so warmed-up preparing for scuba that I only wear a tee shirt.

I also get a little cold snorkeling (I get cold easily). I like to wear a splash top or rash guard shirt over my swimsuit top. It keeps me warm and it blocks the sun. And the rash guard is nice for boogie boarding, too.

was just in hawaii for 2 1/2 weeks and needed to purchase a long-sleeved neoprene for surfing. was fine the last 2 trips also taken in March, but found the water to be much cooler this year (lots of rain & cooler temps)

You won't need wetsuit, but may want a rashguard or neoprene top. You can always decide once you are there...

have fun! aloha!

I will say that you probably will need a wetsuit for the scuba, depending on your tolerance. Water temperatures around that time will likely be in the high 70s. Repeated dives will chill you more.

However, I know people who dive all the time here in just their swimsuit. Personally I don't know how they do it, as I require a minimum of a full 3 mil in the height of summer - but it depends on the individual.

You need to assess the water temperatures to which you're used to and what you wear for that to get a good idea of how thick a wetsuit you may need in Hawaii.

You will need a wetsuit if you are staying at the Marriott on Kauai.
8-)