The sea mines are large naval weapons intended to be used to damage or destroy military vessels, such as submarines. The presence of undetonated and abandoned sea mines suggests that navy forces (likely the American Navy) had passed over these parts of the ocean during a certain time period; the mines may have been intended to stop Japanese naval activity during the second World War. Judging from the state of decomposition, it can be assumed that the mines found in the game had been abandoned for a long time.
Currently, the mines do not seem to have a purpose other than to serve as aesthetic features. They are also harmless to the player and cannot be set off.