If it's the green pod things, you place them in the water with the tip above the water. If it's a proper seed, you need to sprout it in moist soil.
If you bought "black mangrove" it's a proper seed, if you bought "red mangrove" it's a propogule that should be grown emersed. If you plan on growing them in salt water, red mangroves should be started in salt water. I'm not sure if the same applies to black mangroves.