Adding submarines was a stroke of genius and really enjoyable to play, especially hunting other submarines!
Like all new ship classes it needs development to ensure it fits into the overall gameplay. I tested driving a Battleship, cruiser and Destroyer and note the following:
It added a small amount of additional danger to fighting a battleship and it maybe that once the final submarines version is launched that a BB driver can coopt a destroyer or light cruiser as an anti submarine escort. Something would have to be done about shared rewards or such like from this or provide escorts with points for preventing a successful submarine attack.
Submarine hunting was most enjoyable in a destroyer or cruiser but needs a bit more development to really nail it. It might also be interesting to work out a way for more than one surface vessel to hunt a submarine in coordination as was often the case in WW2.
From a submarine fighting perspective, it could be an idea to include a scenario where submarines can attack a convoy of merchantmen while cruisers and destroyers try and defend the convoy. That could create a really good running battle between surface ships and submarines.