6.
There is no change in tax-rate (i.e. 12%) if service-recipient is 'Central Government, State Government, Union territory or a local authority'.
However, tax-rate has increased from 12% to 18% w.e.f. 01.01.2022 when service-recipient is 'a Governmental Authority or a Government Entity'.
Subject of the query says service-recipient as 'Water & Sewage Board' wherein details of query states that 'Water supply Projects being executed for Municipal Corporation (Local authority)'
Now, generally speaking, such types of board does not fall into category of 'Central Government, State Government, Union territory or a local authority'. And they typically falls under 'a Governmental Authority or a Government Entity' who in-turn work for 'Central Government, State Government, Union territory or a local authority'
I suggest that one must first decide 'who is service-recipient and status thereof' before continuing with tax-rate @ 12%. If service-recipient is 'a Governmental Authority or a Government Entity' (& not a local authority), then tax-rate has increased to 18% w.e.f. 01.01.2022 onwards.
Above views are strictly personal and same should not be taken as professional advice.