A beach wedding pulls in two directions. The venue is casual by nature, the weather is usually hot and sticky, and yet it's still a wedding and still asks you to look like you made an effort. A full formal suit is wrong, uncomfortable and faintly absurd on sand. Beach casual is worse, because it tells everyone you didn't take the day seriously. The answer sits between them: a linen or linen-blend suit in a light colour, open collar, no tie. Properly made, worn easily.
Linen, and why nothing else really works on sand
Linen is the fabric for this. It breathes in direct sun, it has a slightly relaxed surface that belongs outdoors, and it looks chosen rather than settled for. A man in a well-cut linen suit at a beach wedding looks like he thought about it, because he did. Nothing else does all three of those jobs at the same time.
Keep the colour light: undyed natural linen, ivory, white, pale grey or sand. Those read festive in outdoor daylight and sit naturally against sea and sky. A navy or charcoal linen suit on a beach looks overdressed in a way no amount of quality will rescue. It isn't that the suit is bad. It's that it belongs at a different wedding.
Accessories and shoes: what to add and what to leave at home
Keep it minimal. A linen or cotton pocket square is welcome; a tie isn't. A fine linen or cotton shirt with the top button open is the right shirt. Shoes are the real problem, because Oxfords look wrong on sand and bare feet look wrong on a groom. The best answer is a good leather loafer worn without socks, or a leather sandal made to the same standard as the rest of the outfit.
If you're in ethnic wear, the equivalent is a fine linen or cotton kurta in a light colour with a churidar or a soft trouser. It reads correctly at an Indian beach ceremony and it's far more comfortable than anything carrying weight or embroidery. Save the silk for the evening event, if there's one.