This will be a mission. There's likely to be sand textures hidden all over the place.
Some will be be embedded into '.ydr' files & such, like this one:
Note: you can use OpenIV's Model Viewer to extract them (look bottom right after opening file. Hint: '[ Export enbedded textures ]') but afaik you'll need to use Texture Toolkit to put them back into a 'ydr' after you have edited them & it can be a right pain in hoop crashing all the time. Stick at it & it should work though.
Best advice I can give you is to use Ctrl+F3 in OpenIV to search for beach/sand/sea related terms & check the '.ytd's for the textures manually.
Searching for terms like '+hidr.ytd' & '+hi.ytd' may also be another way but you'll be dealing with literally thousands of files to check.
These three files have sand textures in them but there will be many more to find than that if you want to edit them all.
Another thing that might help, given you are going to be dealing with so many files & file locations, is to start documenting where you find each file & what it has in it. That will make the whole thing a lot more managable.
Note: Remember most textures have at least two different sizes. What turns up in game will depend on your ingame menu Texture Quality settings. If you plan on releasing it as a mod you will have to find & edit all the textures of various sizes/quality so that all those who download it will be able to see the changes no matter their ingame Texture Quality settings so check similarly named non+hi '.ytd's also when you find them (often, but not always in the same folder as their high quality +hi counterparts).
Sorry, I can't be more positive but you need to know, depending on what you plan to accomplish, this may well take the rest of your life to complete .
I do however wish you the best of luck & am sure that if you are determined enough, whatever you have planned, can be done.