I doubt that you will get the water pressure you want to flush the engine. Try it!
I would be concerned about that also.
The flushing need is more about volume and not pressure. When a washdown pumps says is produces 60 psi let's say, that static with no flow, In addition, all lengths of hose, fittings and faucets add loss to the actual pressure'
On MyOtherBoat, the motors at idle run on about 1 psi of water pressure. At cruise, the pressure may be 6 or 7 pounds. My point is that it doesn't take a lot of pressure to get the water circulating.
On Yamis if the pee stream is showing, you probably have sufficient volume to flush but I have to add that the pee stream is not super accurate indicator for all engine models.
One 'fun' experiment is to remove a thermostat housing and then flush the engine using the flush port and a garden hose. In some cases, you will not see water flowing out of the t-stat opening. So although you think you are flushing, you may be missing some of the more critical areas.. It's very much engine specific.