Many images can be associated with beaches including unique seashells, lush green palm trees swaying in the gentle breeze, soft sand under your toes, gorgeous blue waters, warmth of the bright sunshine, waves crashing against the sand, big brightly colored flowers such as the hibiscus, etc. Luckily, there are invitation styles to fit any of these themes.
Tropical can also relate to the colors used on your wedding invitations. Blues for the water, tans for the sand, greens for the palm trees, bold colors for the tropical flowers, yellow/golds for the warm sunshine.
I think you get the picture and with your tropical invitations you can visually convey your theme to guests.