Hi,
Dry & wet stowage referes to the ammunition stowage with in the tank.
In the early Shermans the ammunition was simply stowed in dry racks and as the Shermans armour was susceptible to penetration by 75 & 88 mm AT guns, was and did quite regularly lead to catastrophic internal fires as the ammo was hit by hot shrapnel.
One method of increasing protection was to retrofit applique armour over the armour stowage in three positions on early Sherman hulls and on the turret cheek this helped but was by no means totally effective.
The other method used on new build Shermans was to fit wet stowage bins where the ammo was stored in a water glycol mix that extinguished fire.
I stand to be corrected but the wet stowage was I think only fitted to hulls with a 47 degree front angle with the larger hatches.
I'm only just learning about Shermans myself but they are so addictive.
Happy modelling
Ant