Building work on concrete in a very wet environment - natural spring
You want hydraulic concrete.

Hydraulic concrete is a concrete that will bond to other surfaces when wet, and cure when wet.  It can even be used underwater.

This page will give some guidance on how to apply.

Given the nature of hydraulic cement I'm thinking that your best bet is - in a dry place, build something that you can put in place to divert the water. Test it by "dry" (haha) fitting it against the spring (if thats the word), and once you're satisfied that it will divert the water properly once sealed, clean the wet space (Pressure washer, lose the moss and stuff) then seal in your diverter system with the hydraulic cement.