And ICBMs, too.
I prefer the idea of submarines, though. There were several cases of very serious incidents involving planes losing control and crashing while carrying nukes, at least with submarines the collateral risk on population is much less.
Water is a very good neutron regulator for fission reactions and general radiation absorbant. Not sure how it deals with fusion though (given that it contains hydrogen and a nuclear blast can probably rip apart water molecules pretty handily).
