Ah, the age old question.... I get this quite frequently and see it on message boards all the time. Which wetsuit to wear can depend on many things - water temperature, body size and type, cold tolerance, how much you are scuba diving, etc. Every person is going to be a bit different, I'll try to give you a basic idea.
When I first moved here it was February and about 74 degrees in the water. I wore a 3/2 mil wetsuit (3 mil on the core and 2 mil on the arms and legs) and that lasted all of 3 dives - then I went to just wearing a swimsuit for quite a while. 74 was quite comfortable. Now that I've been here and diving all this time I'd be wearing at least a 4/3 full suit with a 3 mil shorty over the top to make it through a couple of dives at that temperature. I'm looking at moving to a 5 or 7 this winter if it gets as cold as last winter.... but it's taken years to get to where I need that.
Kona's water temperature on a typical year can run as low as 73/74 during the January through March time frame, then start rising in March through the summer with a high of 81/83 coming in the early fall. This last year we were colder in the winter than usual, seeing 71/72 'til about late April, currently we're running 77/79 or so. At any rate, the need for a particular thickness of wetsuit can vary depending on the time of year.
Most visiting divers here can do OK with a 3 mil full much of the year except during the colder times of the year. Slender divers or divers with a bit of cold intolerance might want something more year round, especially in the winter months. Heavier divers or divers with good cold tolerance may find themselves quite comfortable with less. I've recently started to make the switch to 4/3 full suits in my rental gear for our boat charters. It's just a bit thicker so it takes some of the chill off that might exist with a 3 mil suit. We try to keep shorty suits available for over the top for slender or smaller divers (women tend to require a bit more neoprene than men in general, many might want a 5 mil much of the year) just in case the suits we have or the customers bring just aren't quite enough. Those tend to come out quite a bit in the winter months.
This is pretty vague. If in doubt, you can always go a tad bit thick on the neoprene, a mil or two extra isn't going to be overwhelmingly hot. For the most part, we don't see a lot of visitors in 7 mils. You might see a good number of local DMs and Instructors in 7 mil suits, maybe even a hood or hooded vest in addition, but if you're diving 200-300 days a year 76 degree water can feel like 68.
One note - if you are looking at snorkeling, throw this all out the window. When you are snorkeling you'll probably need nowhere near as much neoprene. You're on top of the water, back exposed to the sun and 85 degree air, and you're typically working a bit harder. Good diving is more controlled floating than anything and you'll be fully surrounded by water (which conducts heat away much faster than air) without the benefit of sun on you. Most visitors often can comfortably snorkel without a wetsuit, or with a shorty or hooded vest only.
I hope this helps some.
The photo above is of a Lei Triggerfish.