Recreational diving at most Finnish wrecks and other underwater sites does not require a separate permit. However, underwater ancient monuments, such as wrecks that sank over 100 years ago, as well as old underwater structures, such as fairway obstacles, the remains of piers and bridges and fishing equipment, are protected under the Antiquities Act (295/63). Particular care should be taken when anchoring at these sites, and the site or the items belonging to it must not be damaged.
Five wrecks protected under the Antiquities Act, namely the Vrouw Maria, St. Mikael, St. Nikolai, Gråharun and Huis te Warmelo, are surrounded by protected areas where diving and anchoring are prohibited. The aim of these restrictions is to protect the underwater sites for future generations and investigations. The Finnish Heritage Agency may grant research permits for these wrecks based on applications.
For sunken warships that are the property of the Finnish Defence Forces (under 100 years old) you need a specific permit to dive from the Military Museum: https://sotamuseo.fi/en/permit-services
The Helsinki Underwater Park at the wreck of Kronprins Gustav Adolf has separate rules concerning anchoring and diving. The location of the park is very susceptible to wind, due to which diving is prohibited when wind speeds exceed 6 m/s. The rules also define maximum size for vessels that can be moored on the buoy. Vessels must not be left unattended. The rules for the underwater park can be found at https://www.museovirasto.fi/en/cultural-environment/archaeological-cultural-heritage/underwater-cultural-heritage-in-finland/helsingin-hylkypuisto
Diving in areas protected by the Finnish Defence Forces requires a permit (see http://puolustusvoimat.fi/suoja-alueluvat (in Finnish)).
Movement in nature reserves may also be restricted (see http://www.metsa.fi/web/en/permits-for-research-and-photography).