Designing Kitchens and Bathrooms - Shower walls

This is a job that was started a while back but never finished by my ex ! He set up individual marbel shower walls and floor ! He used silicone to fill the edges and seems ! I am attempting to removed the silicone and I am finding the spaces are uneven and in one corner theres a good 1/2 in gap ! On the front floor of the shower he stood a piece of the marbel about 3 in. tall and just siliconed it to hold in place so that the water doesnt get out on the floor ! My question is what should i use to fill in the gaps and make it water proof ? should i use bathroom caulk or grout ? There are some pretty good gaps here that need filled in should i caulk then grout maybe ?
HELP !!!

Welcome to the forums! Generally when you order marble wall pieces, they fit pretty good, without large gaps. So we can see what you are seeing, would it be possible for you to post a couple of pix on a site such as photobucket.com and copy/paste the HTML code to your reply post? We may be able to give better advice on seeing your dilemma.