I think submergable cruisers (scruisers) have a few disadvantages compared both to normal ones and submarines:
- Tonnage loss: Adding submarine parts to a cruiser will probably result in about 20% tonnage lost for those parts. Compared to a normal cruiser, the scruiser therefore has less firepower or speed or armour available.
- Vulnerability: If a normal cruiser gets hit, all it loses is the hit part, or complete loss if hit unlucky. If a scruiser is hit, its pressure hull is punctured; it cannot dive anymore.
- Inefficiency: What is the tactical role? As a cruiser, it should attack with a fleet (at least for us, cruisers are our battleships). As a submarine, it should attack convoys or do sneak attacks on the enemy fleet. For the former, the submersibility (nice word) is not needed, for the latter, its firepower is unneeded and the risk to the ship itself is high (see vulnerability).
I therefore believe that a scruiser has less firepower than a normal cruiser, once hit loses its submersibility and sits between two chairs for its role.
A submersible aircraft carrier has about the same problems, though less with vulnerability but with the planes require to find the ship to land, and that stowing them takes time thereby slowing the deployment and undeployment. Plus, it has fewer planes than a comparable surface aircraft carrier (Compare the Hosho to the I-400. Hosho's a bit bigger, but carries five times the planes, and is not restricted to seaplanes).
@Cato: The current solution seems to be a combination of 1 and 5, that is a Raven-modification for the short term and a torpedo bomber complimenting that. But we need one more vote for that.