Almost all cementitious products cure better, stronger and harder with a steady presence of [fresh] water. As far as placing the product, whichever you choose, in the presence of water, that will depend on specifics that you have not listed. I will say that when large construction companies are pouring underwater footings, say for bridges, they will have the form in place and filled with a heavier than water gel that gets displaced by the concrete as it is poured in/injected. If the water is intermittent or tidal you can use a simple fast setting concrete, or a normal concrete with an accelerant additive. If it is saltwater you can use additives suited to saltwater. Your local concrete store will be able to direct you in the right direction as far as additives go. Be careful about what kind of reinforcement you use - many types of reinforcement are very susceptible to sulfates in the water and will cause spalling if placed too close to the outer edge of the concrete.
With regard to your initial description, I would say that most concrete pours end up with a void. As long as no water is penetrating through the void in your wall you likely have no concerns in re.: future erosion. Even if water were coming through this void the concrete has likely formed a crystalline structure that is resistant to "erosion," however if water is coming through then there are many cementitious products available for purchase that will solve this problem. I've used Xypex and at least one other for weeping foundation walls. Insofar as patching the void is concerned I've had good luck with a product called Thoropatch.
Best of luck in your endeavor, even if I'm not sure what said endeavor is...