Concrete is resistant to sea water. Coastal bridge piers are designed and constructed without special protection.
Check my website, link below, and download "Concrete in Sea Water" on the out-of-print Portland Cement Association page.

www.SlideRuleEra.net

As additional information to the reply frm SlideRuleEra:

Concrete in Sea Water? Then you have to consider the potential of corrosion by chloride content in sea water (for the reinforcement)! In general you have to take a concrete with higher strength category.

wrapping the concrete with PVC is not sufficent because the conjunction of PVC and concrete is a problem.

Regards
Bartz

Is this pier and footing going to be cast in place, or precast? It will not be so easy to construct this to a good standard. Could you drive a concrete precast pile and then form a pile cap abobe the water level.

If you are going to buid a traditional foundation, you can do that with concrete. Many such structures have been built and have worked very well. However, the foundation must be built in the dry. This ussually done inside a sheet pile cofferdam. Often underwater (tremie)concrete is used to seal the dam, but it is not used as a structural element in the foundation. Although the concrete can be used as a bearing surface for the final structure. This type of construction can be quite expensive and thus the piles mentioned by Zambo are often a good option.

You can pour under water using Fabriform Nylon bags. They are  used for bases of bridge foundations. Bags are relatively cheap easy to use and can be placed over an uneven base like  a rock and will fill to give you a flat foundation to build your piers on. You can also insert rebar into the bag,(epoxy coated rebar]to provide a connection to your vertical supports. It is also possible to provide a fabriform pile jacket forms  for your vertical piers supports.

David Ritchie, Intrusion Prepakt

Intrusion Prepakt
www.marineconcrete.com