Griff is right. You must measure the oil temp in the system where the oil is the hottest and usually that's in the pan or just before the oil cooler or oil cooler thermostat.
We never start our dyno pulls until the engine oil reaches a minimum of 165-170 degrees and in a boat if your temp was 180 degrees or so at idle and low rpms under light load that would be a good starting place and yes I think about 260 degrees on the high side is plenty after hard runs especially if you are just running conventional oils.
Synthetics can take higher temps, but even at 290-300 degress or so they start to loose to much viscosity and lubricating quality. Its very correct that too cool of oil temps will build moisture in the crankcase (condensation) very quickly in a marine engine and the resulting water in oil damage can really wreak havoc on the internals, especially bearings.
Ray @ Raylar