There are many places to look and it really depends on which specific area of WASH you are interested in.
Generally speaking Josh's Water Jobs
has a very good listing for international WASH jobs, although their offers tend to be a bit US leaning for the non NGO positions.
For humanitarian work, there is of course ReliefWeb, where you can filter for WASH jobs
Most international NGOs also run their own vacancies pages and there are some semi-headhunting firms, but they tend to have a lot of offers where consultancies are asking for CVs to submit as part of proposals... which really isn't a job offer (yet) at all.