In terms of its salinity and water quality, the Archipelago Sea is the Finnish coastline's most marine area. Finland's remaining healthy underwater ecosystems flourish there. A broad cluster of over 40,000 islands, it is one of the Baltic Sea's most unique areas.

Due to its island density, the Archipelago Sea has a low average depth, at 23 metres, compared to 60 metres for the entire Baltic Sea. Surprisingly, despite its modest depth, the Archipelago Sea is akin to a miniatyre underwater mountain range; it is full of long depressions over 60 metres deep, the deepest reaching over 100 metres. The bottom often plunges downwards just off the coast. Due to these depths, most water circulation between the Baltic Sea and the Gulf of Bothnia occurs via the Archipelago Sea.

No one has a complete picture of the Archipelago Sea's underwater landscape. One must seek out scenic photography locations and thriving ecological communities for oneself. In many cases, a prime landscape subject will measure only a few square metres. Many sites may never have been visited by divers at all, let alone photographed.