In early July the water temperature here (St. Thomas/St. John) will be in the mid 80's... probably around 83F to 84F. I use a full 3mm wetsuit in the summer which is a little over kill at the peak, but I hate to get chilled. I have seen plenty of people with no wetsuit at all, even during the winter months. Our water temperature ranges from a low of around 77F in winter to a high of around 85F in summer. Plan for it to be 84F and you won't be far off.
> Is a thin shorty doable
Very doable. I would recommend this over a 3mm unless you get chilled easily.
> I know DMs down there that wear 5's most of the year.
I cannot imagine wearing a 5mm full wetsuit during the summer months down here. I have to break the neck seal on my 3mm to cool off from time to time during the hotest summer days. I don't doubt that you know some that do wear a 5mm all season long, but my guess is that they would wear less if it were freely available.
> wear a hood
A hood during the summer here is overkill, though it is nice to have during the winter months.
