Raw Water Housing Change out procedure

[SeaAyeOwe]

Guys - my raw water housings on the 350 mag's are the metal type and are rusted with small leaks so I am going to change them out.
I have bIII's.
Lifting out is not practical right now.
An ask for anyone that's has done this job in the water - I would like some advice.
I think the housings are below the water line, I can of course close off the sea cock but will the water flow back in through the BIII water intake?
Any advice on ten correct isolation and remove install procedure is appreciated.

 

[SeaAyeOwe]

Ok - owners manual is not 100 % Clear to me - that have a section "Boat in Water" and a section"Boat Out ofWater" - but for closed cooled sea water exchange boats both sections are identical.
Unless someone tells me otherwise closing the seacock and using the air pump then removing the housing is all I see to do.