Lifespan of underwater sensors
Temperature, turbidity and conductivity probes that can be continuously submerged are not very expensive, but pH is different.  pH probes are made of glass electrodes that exchange ions with the surrounding media.  Usually the probes are only briefly exposed to the media to avoid fouling and minimize how gain/loss of ions through the glass.  Even then, you'll typically need to recalibrate the probe over time as it gains/loses ions.
Do you really need continuous pH monitoring over such a long time?  Can you periodically replace or recalibrate the probe?  If not, there are vendors that can do that, but expect the probes to be much more costly than those that are only expected to be briefly exposed to the environment and to only hold a calibration for a short time.