In example, we love the T-shirt (look at the uploaded pictures, the first one) with the wreck in the back and the diver in front! but would have to be adapted to our huge Cozumel reefs.
I give you here some concepts as a guideline for the design :
(the uploaded pictures are there for inspiration, not to be copy and paste in the design...)
+Diving spirit : references to adventure, exploration, mystery, encounter, wonders...
+Cozumel Marine life superstars : Sea turtle (look at the uploaded pictures).
+Don't be afraid to use all the T-shirt surface (look at the uploaded pictures).
+Use design with dynamic, movements and perspectives (look at the uploaded pictures).
+The logo can be on the heart, discrete OR in the back, small under the necklace OR in the bottom on a side front or back.
An idea? => Perhaps a diver in front with a reef environment in back landscape and a huge turtle head behind, looking at him. Like a ghost or coming from the shadows.