COVID-19 negative certification required for entering the country.
From 20 February 2022 there is no PCR testing nor quarantine requirements for any arrivals regardless of their vaccination status, including those arriving via the Causeway.
Your airline may have their own boarding requirements and may be liable to move to a reduced service as appropriate depending on the COVID-19 situation. You should check with your airline for the most up to date information before travelling to Bahrain.