The most suitable cement for use in underwater applications is 

Concept:
Sulphate Resisting Cement (SRC):

• In this cement, the amount of tricalcium aluminate (C3A) is restricted to an acceptably low value generally less than 5%.
• It is recommended for use in aggressive sulfate-rich environments where the concrete is exposed to the risk of deterioration and spalling due to sulfate attack.
•  SRC should be used in concrete used for the fabrication of pipes that are likely to be buried in marshy regions or sulfate-bearing soils because it has the risk of deterioration due to sulfate attack.
•  SRC should be used in concrete to be used in the construction of sewage treatment works because sewage water often contains an excessive amount of sulfates.

Additional Information Low Heat Cement: 

• This cement is manufactured by reducing the proportion of C3A and C3S and increasing the proportion of C2S. This cement shows a low rate of development of strength (Due to increasing C2S). This cement is used in mass concreting.
• Example - Hydraulic structures.

Portland Pozzolana Cement: T

• his cement is manufactured by inter-grinding cement clinkers with (10 - 25)% pozzolanic material.
• This cement is found to have a low rate of development of strength, therefore used in mass concrete construction.

Portland blast furnace slag cement: 

• This cement is produced by inter-grinding cement clinkers with gypsum and granulated blast furnace slag cement in a definite proportion.
• This cement possesses high resistance against the attack of chlorides and sulfates. It offers high water tightness as it has reduced permeability due to the refinement of the pore structure. Therefore this cement finds its application in the mass concreting of hydraulic structures