yes, you can do it with the boat in the water
yes, you will get water coming into the boat
have a helper handy.
get a section of 1-1/4" dowel rod and cut off 2" and round the corners
undo the hose from the back of the water pump running to your oil cooler and stuff in the chunk of dowl rod and tighten the clamp.
now you wont have water spraying in on you while you change the pump
the screws are brass, so dont loose them.
you will need a pliers to pull out the old impeller
if you lost bits of your impeller, buy a new one
if your pump is bad, buy a new one. its a Johnson F5 pump.
https://www.google.com/search?client=firefox-b-1-d&q=johnson+f5b-9+pump
however if you havent pulled the boat to do some other drive maintenence. you should. Bellows every 3 years. raw water inlet fitting replace every 3 years (the one on top the H-frame)