Clusters of charged groups on the surface of proton-transfer proteins may participate in proton transfers. PsbO, an extrinsic subunit of photosystem II, is a carboxylate-rich protein part of an extensive hydrogen-bond network leading to the catalytic site. This raises the important question as to whether specific clusters of carboxylate groups on the surface of PsbO may directly assist transfer of protons from the catalytic site to the bulk. From molecular dynamics simulations of PsbO in aqueous solution, we find that, close to the surface of PsbO, the mobility of water molecules is lower than that for bulk water. At the site where PsbO docks and hydrogen bonds to photosystem II, water molecules have low mobility, and we identify a carboxylate cluster with persistent bridging of the carboxylates via short hydrogen-bonding water wires. This water-bridged carboxylate cluster could be important for proton transfer and binding of PsbO to photosystem II.