The sea area in question is a Marine Protected Area, which is referred to in the united-kingdom as a Marine Conservation Zone (as per No17 on the 2016 linked list) and lies within the UK's territorial waters.
What UK law prohibits this?
I have discounted my initial thoughts about s.58(2) of the Merchant Shipping Act 1995 as I strongly suspect the Greenpeace vessel is neither UK registered nor proceeding to/from a UK port.
I have also considered the Criminal Damage Act 1971 which, although potentially viable* and has off-shore jurisdiction, I have discounted in favour of the Marine and Coastal Access Act 2009 which creates the offence of depositing objects in the UK's territorial waters without a licence:
s.65 Requirement for a Licence
(1) No person may -
(a) carry on a licensable marine activity
except in accordance with a marine licence granted by the appropriate licensing authority.
s.66(1) Licensable Marine Activity includes:
1.(a) To deposit any substance or object within the UK marine licensing area, either in the sea or on or under the sea bed, from ... any ... vessel ...
s.66(4) states the "UK marine licensing area” consists of the UK Marine Area which is defined by s.42 to include:
the area of sea within the seaward limits of the territorial sea adjacent to the United Kingdom
What is the penalty?
s.85(4) states anyone guilty of a s.65(1) offence is liable:
(a) on summary conviction, to a fine not exceeding £50,000,
(b) on conviction on indictment, to a fine or to imprisonment for a term not exceeding two years or to both.
*relating to the seabed as well as the bottom trawlers.