There are a number of water fountains throughout the terminal –
At the Assistance Area before you go through security and also beyond security:
Within the main departure lounge, behind Bottega Prosecco Bar & Cafe
Within the main departure lounge, adjacent to the toilets behind Number 1 Lounge
International Pier by the departure gates
Pier Bussing lounge
North Baggage Reclaim area – Arrivals
Alternatively you can request tap water at any food/catering outlets.
From the Airport twitter feed
#plasticfree @bhx_official