Get a big bucket. You can just take a plastic hose connector
stick it in the raw water hose that you took off the strainer (if you have a strainer) or off the thru hull if you don't have a strainer. Take a length of garden hose or any other hose that is long enough to make your water hose reach the bucket. Fill the bucket, (carefully fill the hose) and then stick the hose in the bucket full of water. (Filling the hose is "priming it" That way your engine water pump is not sucking air, it is sucking water.)
Have someone on the water hose that is from the tap going INTO the bucket to keep the water level high enough so the engine continues to suck water.
Do not put the engine in gear!!!
When it starts, check over the back to make sure water is coming out. If not - shut down and check everything.
When you are all done running the engine, turn off the tap and reconnect everything the way it is supposed to go.
Put your hose connector and the extra length of hose in the bucket, take it home and then you will have all the parts you need for next time.