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Decreasing Oil Pressure
Generally, low oil pressure is an indication of bearing wear. The engine oil pump can pump a certain volume of oil which is distributed to the various bearings in the engine. As these bearings wear, they allow more oil to flow past them due to a greater clearance between the bearing and the surface the bearing operates against. Eventually, the oil pump is unable to pump a sufficient volume of oil (because of the leakage) and the oil pressure begins to fall.
Oil pressure normally falls as the oil reaches a higher temperature and becomes thinner, but when it falls below the manufacturers oil pressure range it usually means excessive engine wear.
If you are lucky, it could be the oil pressure relief valve is stuck in the open position which can also cause a similiar indication. Unless the engine has been abused, 900 hours is not normally enough time to experience excessive bearing wear.
An evaluation by a good boat engine mechanic should be done before you accept the worst case scenario.