why fix what aint broke?
Surface ships can blockade for 1 point of IPC , while Subs can raid for 2, seems pretty simple to me.
what you could do is keep surface ships the same, they do one point of IPC damage per ship, though perhaps make it so they have to have atleast one of each surface ship (Battleship, Cruiser, Destroyer) to do so. Represents actually setting up a task force to blockade someone, and it would do 3 points of damage.
For subs you could keep it the same, but make it that German subs do 3 points of damage, to represent the wolf packs of WW2. Another thought would be allow subs to do convoy raids like SBR, but allow German subs to roll a D6 for damage while other nations roll D3. Of course you’d then have to change the cost accordingly and that would make it more complicated.