Re: repairing diesel engine on the dry
First look at your engine's tech manual for the raw water pump specifications - rpm vs. gallons per minute. Get a large tub (about 10 gallons) and put it into your cockpit' and adjust the water (garden) hose inflow to the tub to be approx. that needed, any spillover from the tube will empty through the cockpit drains. The goal is to keep the tub full when the engine is running.
Most Diesel boat engines in the 30 hp range will need ~3 gallon per minute for running at
Run/extend a hose from the raw water pump to the bottom of the tub, completely fill the hose (no air); start the engine; watch/monitor (need assistant) the level in the tub so that you 'have enough' in the tub at all times.
A cheap plastic mason's tub (Home Depot, etc.) will be in the 10 gallon size range.