Here are a few things that you will want to consider...
Sun Block. Waterproof, sweat proof, and 30+ SPF. You may want to consider zinc oxide (or titanium dioxide), especially for face, nose, ears, and lips. Some zinc oxide sunblocks now are available will go on clear and still offer a high degree of protection. 
Clothing and the Sun. A typical cotton t-shirt only does not necessarily protect you from the sun. Sometimes you should still put sunblock on for full protection. There are some clothing companies that offer clothes which have had their sun protection factor (SPF) measured (some even have measured the UPF - ultraviolet protection factor). If you plan on relying on your clothing for sun protection, your search should start with clothes that start at 30 SPF (blocks 96 - 97.4 % of UV rays according to the American Melanoma Foundation). (pictured: Columbia Sportswear Skiff Guide Long Sleeve Tee, 30 SPF)
Hats, Gloves, Etc.. Another essential piece of equipment is a hat. The more brim the better. Baseball cap style is your starting point, but wide brimmed hats will offer the best protection and keep you 
the coolest. Surfers might even consider water hats that come with a secure chin strap that will keep the hat on your head when you duckdive or wipeout. Fishermen might want to consider hats that have integrated neck protection and long brims to allow for maximum visibility when sight-fishing. Many serious fishermen even go so far as using "sun gloves" and "sun masks" that are made from lightweight and breathable materials that are surprisingly comfortable, even in the tropics. (Pictured: Dakine - Indo Surf Hat, Patagonia - Sun Mask)
Staying Dry. Keeping your clothing as well as yourself as dry as possible is important in the humidity of the tropics. Always do your best to keep a dry towel around at the end of the day and pack a small container of talc powder to help treat minor skin irritation caused by moisture.
