Captain reported aircraft failed to pressurized correctly on climb and elected to perform an air turn back and precautionary landing at departure airport.

Narrative

The report is to document an inflight incident involving the aircraft pressurization system which subsequently lead to return to field. The problem initiated during the climb phase of flight on the ZZZZZ departure. We were climbing through 16;000 feet to the assigned altitude flight level 230. I began hearing a high pitch noise which started out as faint. ZZZ ATC center controller cleared us to climb to FL340 and direct to way point ZZZ1. As we continued to climb; the high pitched noise became noticeably louder. The First Officer and I began to discuss and investigate to see if the source of the noise was in the communication system or elsewhere. I briefly removed my headset and noticed that the noise much louder. We deduced that the problem was not in the communication system but somewhere else we could not identify. We began to use our SOPs and tactics to troubleshoot the problem. We considered the possibility of a leak affecting pressurization. I commanded the first officer continued to fly the aircraft and request from ATC to level off at FL260 to address an issue. At FL260; we slowed to 250 knots. I opened the QRH to Loss of Pressurization section. Running the checklist; we compared the pressurization indications on the EICAS against the table in the checklist. The table stated the following information: 26;000 ft / CAB ALT 4100 / @P 7.4. The press. EICAS indications were higher than the table posted: CAB ALT 6900 / @P 6.1 / CAB RATE 0 fpm. The high pitch noise was significantly louder at FL260 than at the lower altitudes. We checked the pressurization controller which was properly set. We also checked our respective DV (Direct Vision) and forward windshield's for leaks. DV windscreen were properly secure and nothing was noticeable. We contacted the flight attendant to query if there was any unusual high pitched noise in the cabin area. Considering this noise condition and the difference in the pressurization reading; we contacted Dispatch and Maintenance Control via company frequency to aid our troubleshooting. We completed all tasks instructed by Maintenance Control and checked with the Flight Attendant for any leaks at the main cabin and galley service door with no defining discoveries in those areas. At the end of communication with Maintenance Control; the decisions was made to return to ZZZ airport. We were transferred back to the dispatcher to get an amendment for return to field (RTF). At that moment; I received an intercom call from the field attendant informing us of a loud hissing noise in the lavatory coming from behind the mirror (potable water tank location) above the sink. We contacted ATC to advise them our pressurization problem and requested vectors to return to ZZZ and began our descent to 10;000 feet. We received our vector clearance direct to ZZZ and descent to 16;000 feet. The first officer and I assessed that the aircraft status was adequate with stable pressurization reading not requiring the need for [requesting priority handling] and immediate descent. The First Officer continued to fly giving me greater latitude to manage the situation. I contacted the Flight Attendant to complete a briefing for our RTF to ZZZ and He read back the instructions. I briefed the passengers the nature of our abnormal situation and intentions to return to ZZZ. We received the amendment for RTF from dispatch via ACARS. We reprogrammed the FMS; setup navigation instruments; conducting Weight & Balance performance calculations; and completed checklist/ briefings for our landing. ATC cleared us continued descent to 10;000 feet. The Flight Attendant call back to inform us the passengers were briefed and cabin was ready for landing. I was able to contact ZZZ station operations to inform them we were returning to the airport. After a thorough briefing and giving due consideration to the current situation; we agreed to transfer control of the aircraft and I would complete the landing. We vectored around some reported convective cells continued flight to without further incident. We observed at approximately 1;000 feet prior to landing; the cabin made a loud squealing noise during depressurization. We landed on Runway XXL and taxing to gate XX. Upon arrival at the gate; we shutdown the aircraft and completed all required checklist. After the deplaning of the passengers; I initiated a maintenance write up. During the post-flight inspection; the potable water service cap was discovered uninstalled. The flight was met by the Base Chief Pilot; Company Gate Agent; and Maintenance Technicians. We were able to transfer to another aircraft and continue the flight without further incident.