Is there a shower? Looks like the spray from the shower is running off the cill to the left but the cill probably doesn't have a fall to the front so it just gathers against the left reveal and eventually soaks into the plaster. It'll be fine when it Dries out but it might take a while. As for how to stop it? Move the bath, but I'm guessing it's not an option or it wouldn't be there. Fall the cill to the front or get another shower curtain across the window. You could also chop the cill out, cast it in concrete with a waterproofer in it and tile it.
Is there a shower? Looks like the spray from the shower is running off the cill to the left but the cill probably doesn't have a fall to the front so it just gathers against the left reveal and eventually soaks into the plaster. It'll be fine when it Dries out but it might take a while. As for how to stop it? Move the bath, but I'm guessing it's not an option or it wouldn't be there. Fall the cill to the front or get another shower curtain across the window. You could also chop the cill out, cast it in concrete with a waterproofer in it and tile it.
B.