If it's only when your draining water it's a waste problem get the bath grating and waste checked out you'd need to get a good look to see exactly what parts leaking the best solution is to then replace faulty part that's all I can offer find fault and replace faulty part with a bath it's usually the grating.
get the bath panel off, and check the waste trap (underside connection to bath plughole)
Could be leaking underneath the edge of the seal on the waste as it sits on the bath surface
Could be waste trap connections which should be compression (threaded and sealed connections)
Could be the overflow (usually a flexible plastic pipe that connects into the side of the trap
Test by putting a bit of water in the bath - plug in first of course and a towel or vessel (takeaway trays are pretty good) to catch the leak
We always recomend to post a job on main website because some of a job needs to be seen before any advice is given.
Our service is free of charge with no obligation quote and our tradesmans are happy to help with every aspects of jobs.
I would strongly recomend to post a job on Rated People home page then 3 companies will get in touch then you would be able to choose the right person as you would receive an e mail with a link to tradesman account where you would see a pictures and you will read revious from previous customers.
I hope this will help