Why do you want to block the tank with expandable foam ?
Not safe, and possible chemical reaction could be expected against SS, giving rust and holes.
I had this case in an italian rib, tank under the deck, impossible to remove without cutting it ...!
No PU foam is closed cell, so it becomes a sponge when the water gest around !
It is closed cells when expanded in a closed and tight volume, with a precise volume injected, but not in free expansion.
I prefer to maintain the tank with straps - I add GRP D-shaped, stratified where necessary, into the tank compartment - and eventually I add cheap white polstyrene laminated foam to fill the volumes around