Dive's storyline has your character scuba-diving in the sea around various islands, in order to find mysterious treasures and clues that will lead him to the lost "Temple of the Order of Medes Island." His secondary objective: to find other treasures that can be exchanged for upgrades to his suit, harpoon gun, oxygen tank and flashlight. Once you've found the "main" treasure, you have the option to return to that island or any previous island to find additional loot you missed. That adds a sort of light Metroidvania feel to the game, as you can use your upgraded abilities to navigate previously inaccessible areas. Those abilities include increased harpoon stores for dispatching any sea life that gets in your way, increased air capacity, better diving suits that allow you to reach greater depths, brighter and longer-lasting flashlights, and increased speed.
Unfortunately, those abilities don't really change in any appreciable way as you upgrade them. I had to get pretty much all the way from the lowest speed level to the highest before I could tell that my diver was moving any quicker. And being able to carry six harpoons at a time instead of five isn't particularly thrilling.
With the exception of each island's main treasure, the stuff you're picking up -- the whole reason you're searching every part of these islands -- isn't interesting either, represented on the screen only as a white twinkle that reveals itself briefly as a ring or gem or statuette when you pick it up. That's not much payoff for finding it.
As for the islands, they all look pretty much the same and sport similar marine life, though as the game progresses, more sea creatures will show up. Again, this doesn't provide much incentive for exploration.
(On the subject of that sea life: I expected a semi-conservationist message from a game about the beauty of the oceans, but the game actually teaches players that all aquatic lifeforms are a bunch of aggressive jerks and deserve all the harpoons you can throw at them.)
Dive's problems seem more glaring because I liked it so much initially. If you play through the first two or three islands, I'm betting you'll feel the same way. For about that long, it's a fresh and unique side-scroller that lets you explore some beautiful seascapes, but after that, the air in its tanks starts to get stale.