Actually, the warmer you can keep yourself, the better... even in tropical waters. A 3mm isn't overkill. Wearing shorts (even a shorty actually) and/or a swim suit actually increase your chances of DCS... even in tropical waters.
Water cools your body... unless the water is 38C, it robs your body of heat... and causes your metabolism to slow down.
Now, you don't have to wear a 5mm or dry suit all the time just to keep warm... but I'd stay away from the 'shorts/bikini' outfit... or even shorties for that matter. I personally feel that a full skin or 1.5mm is much better than a 3mm shorty at keeping your body temp normal.
Oh, and another thing, a lot of people forget that a wetsuit only works when fitted properly! A loose 6.5mm suit is worse than a properly fitted 1.5mm suit in cold water. neck, wrist, and ankle seals must be as water-tight as possible to minimize water-flow within the suit, and this is what helps keep you warm.
Although I no longer use Henderson suits, their hyperstretch and black maxx line are great for tropical-temperate waters. They cling to your skin better than most suits and keep you just as warm with a lot less neoprene (so they're easier to don-doff)
Another thing as well, closed-cell neoprene (the shiny type) keeps you a lot warmer than open-cell (looks like a sponge) neoprene, with the same thickness.