If your ceremony is held on a tropical island, sandy beach or private beach, you can go for a really relaxed, casual look like something simple and lightweight.
Whether just a bikini top with a sarong, strapless dress, halter, one-shoulder dress, loose fitting clothes or trousers with a floaty top - consider the beach heat and dress for that.
Beach wedding dresses can be very casual so you don't really even need a veil. A very lightweight shawl draped over your arms can also be a subtle substitute for a veil. This would also be very helpful if later on a cool breeze comes up, you already have a shawl wrap to keep the cold off.
Ideal cuts to go for in the dress are sleeveless, tube top, strapless, and off-the-shoulder since these give the ultimate comfort.
You can even have a strapless gown and just twist a band of translucent lace around your shoulders, tied at the back, for a softening of the decolletage, and hiding your upper arms if you want the comfort of sleeveless, but with a wisp of lace to help conceal any flaws..
Whatever the case, you'll need to think about where you'll be saying "I Do" and what would be best to wear.
If you decide that the wedding is very informal that will make it important to wear something comfortable and casual.