A320 Captain reported receiving an ECAM; HYD Y ENG PUMP PR LO during cruise flight. Due to the possibility of additional fluid loss from the unknown leak; the flight crew opted to land at an alternate airport.

Narrative

We arrived at the aircraft about 35 minutes before departure due to security and customs. During our preflight we noted current MEL items and an inbound write-up. The aircraft had two concerning MELs; the Fuel Auto Feed Center tank pumps were inoperative and the ACARS Printer was inoperative. We discussed the Center Tank Fuel pumps and were not concerned as we were not going to have center tank fuel. The ACARS Printer was a bigger threat as we would have to transcribe all our ACARS information. We would also have to receive the Maintenance Release in paper form. The inbound write-up was for the #2 Pack Flow Control always in LO. There was a Mechanic on board looking at something in the cabin. I asked him if he was aware of and dealing with the inbound write-up. he indicated he would check; but I never heard back from him.As we were finalizing our preflight; I called Dispatch to see if they could find out about our pack write-up. They said they would ask Maintenance Control and call back. They called back a few minutes later with Maintenance Control on the line. Maintenance Control advised that they would just defer the #2 pack if that was okay with Dispatch. Maintenance Control proceeded to go through an MEL procedure. I assumed he was just telling me about it and did not realize initially that they wanted me to complete the deferral process. I asked them if we have Maintenance here and why can they not do this? He blew me off and said we can just walk through this process. I had him hold for a minute while I discussed the concern of losing one pack with my FO (First Officer). He was okay and I knew we could talk about it more if needed. So I found the MEL card; 21-XX-X for PACK 2; followed Maintenance Control's instructions; and we deferred the pack. I spoke with Dispatch again and discussed the new release and the need to have a paper Maintenance Release sent to the aircraft.I finally had a free moment to bring ZZZ Operations into the loop about the status of the maintenance issues and to inform the FAs (Flight Attendant) of why the delay and what impact the single pack may have on cabin comfort. My FO and I then had a detailed discussion about the implications of all these MEL items and the pack in particular. In my mind; I had always reserved the final right to turn down this plane if we felt uncomfortable. We talked about the weather in ZZZ1 and how the pack inoperative may affect us. We felt that there was no present threat with the pack inoperative and the forecasted low ceilings in ZZZ1. We noted that all our alternates; including closer airports than the planned ZZZ2; were good alternates. We also discussed that we had lots of alternates all down the coast in the event we had any pack related complications. We decided the fuel MEL would not be a labor intensive and the ACARS Printer was more labor intensive; but not a concerning threat. We came to the conclusion that we would go once we got the Maintenance Release.We started up and normal taxi and takeoff out of ZZZ. Normal climb and level off at cruise FL290; for turbulence above. As we passed over ZZZ3 and were continuing to the south; we got an ECAM; HYD Y ENG PUMP PR LO. It was my leg; I said I have the aircraft and the radios and the FO completed the ECAM. We noted the low level in the yellow system and the inoperative PTU (Power Transfer Unit). We assessed our options; continue to ZZZ4 or turn back for ZZZ3; ZZZ1 was not a consideration. ZZZ4 was about one hour and twenty minutes away with comparable weather to ZZZ3. We transferred the controls and comms to FO. I called Dispatch to bring them into the loop and see what their recommendations were. I discussed the issue and they recommended ZZZ4. I told them to hold and discussed it with my FO. We discussed the potential for additional fluid loss. We could tell we had lost fluid; but did not know from where. As silly as it may sound; I was not in the mood to lose a second HYD system to a mystery leak. Considering time in route to ZZZ4 and proximity to ZZZ3; we chose to go back into ZZZ3. I informed Dispatch that we considered ZZZ4; but were being more cautious and chose ZZZ3. I informed the flight attendants and then informed the passengers.We then [requested priority handling] with ZZZ Center and proceeded back to ZZZ3. Checklists and landing performance; V-speeds and conditions considered; we chose vectors for a straight-in ILS to a visual approach to Runway XXR. We discussed the approach considering the flaps and the after-landing contingencies. We assessed the tools we had and did not have; normal brakes; nose wheel steering; only one thrust reverser; slow flap deployment; less effective spoilers. The landing was normal - for me! - and uneventful. We declined any assistance on the runway and told them we would be able to exit and taxi to a gate. We declined any further need of the CFR (Crash Fire Rescue) equipment as all was normal for the taxi. We taxied to the gate; parked; and shut down as normal. We had some complication at the gate with deplaning the passengers. The ZZZ3 Station Operations was there to meet us; but they were unsure whether to deplane or what the plan was. Then once they had a plan; they had a computer issue with the gate reader to accurately deplane the passengers in the system. With a few calls to Chief Pilot [and] their help from the Company Communications System; we were able to get the passengers off the aircraft and taken care of. We and the flight attendants debriefed and we deplaned. FO and I discussed continuing for the day and determined that we were safer not continuing. We worked with the crew desk to reschedule.