There are two known Marine Blues, one is found underwater (heart shaped) and one is found on a land (diamond shaped).
Legendary Marine Blue (Sea version)
The heart shaped charm Marine Blue, is found in the deep depth parts of the sea. This little charm can transform any fish or mermaid into a Tamagotchi (able to walk and breathe on land). But this charm cannot be used to transform a Tamagotchi into a fish or mermaid, only the land version Marine Blue can do so.
Legendary Marine Blue (Land version)
The diamond shaped cone Marine Blue, is similar to the sea version of the Marine Blue. But this diamond cone that is found on a forgotten island, hidden in a trapped cave can transform any Tamagotchi into a fish or mermaid. But it cannot transform any fish or mermaid into a Tamagotchi, only the sea version Marine Blue can do so.