DAP makes a good one. https://amzn.to/3acpp2E I used it at our old house. I need to do mine as well! thanks for the reminder!
Caulk is not used to waterproof a shower floor. It's used to seal joints where two different elevations meet. Like in corners where walls meet floors. Grout is used to fill the joints in between tiles and grout sealer applied.
William, you are so knowledgeable!
You want one that is waterproof and labeled for sinks and tubs. I now have to replace the cheap, caulk an installer used in my new house. Not an unpleasant job, just irritating seeing it break down. Caulk, good caulk isn't that expensive. Caulk is used to seal the seams, not the gaps between the tiles, so if you are missing grout, it's a different process and material.
I would purchase the exterior version as it is designed to withstand the most exposure to water.
There is a version that is made specifically for tub and tile that should work well also.
I am curious where your application is going to be.
Here is an idea for you about caulk https://www.homegearkit.com/best-caulk-for-shower/
Any of these will be fine for your shower..