Oh I know...I know!!!!!

That additive is.....

(drum roll please!!!)

Cement!

And yes, I am reasonably certain my local concrete company uses it!

(Oh this is so much fun....but really dude...you set yourself up for that one! Concrete will set up stronger and better in the wet...in fact, it works best when poured under (fresh) water. It doesn't get weaker in the wet, as your question seems to imply)

Well, that's sorted, then.

How nice, he came back to share his new found knowledge.

Jerry, it's takes a high degree of cement to make concrete stronger, as well as careful proportioning of pozzolans (if used), fine and coarse aggregate gradations, a lower then normal water-to-cement ratio, and careful and proper curing techniques & durations.

I suggest that you go procure (for free) several U.S. Army Corps of Engineers (USACE) Engineering Manuals (EM 1110-X-XXXX series) dealing specifically with concrete in general as well as the use of High-Strength Concretes.

Yes, you can use Rapid-Hardening Cements such as Magnesium-phosphate cement (MPC) and High Alumina cements (HAC) for high early strength, or even silica-fume which is a fine amorphous pozzolanic material.

I know of no additive that will make a concrete mix stronger in the presence of water. In fact, the more water you use in a mix the more it will weaken the concrete, hence lower it's strength properties (Compressive Strength, Flexural Strength, Tensile Strength, and Modulus of Rupture.....)

I can supply you with the proper link to the USACE website that contains the EM series that you would need. It is then up to you to download the appropriate EM and actually read it as well as fully understand it.