Firstly: The Ruling of Killing Game while in Ihram
Killing game is restricted in ihram. Consensus on this was related by Ibn al-Mundhir, Ibn Rushd, Ibn Qudamah, al-Nawawi, and Ibn Muflih.
Secondly: What Game is
One in a state of ihram is restricted hunting wild, terrestrial animals whose meat is permissible to eat. This is the position of the Shafi`i and Hanbali schools. It is also the position of Ibn `Uthaymin.
It is permissible for one in ihram to kill marine life and eat it. Consensus on this was related by Ibn al-Mundhir and Ibn Qudamah.