In my opinion Lipos and even more expensive types are great for helis,cars ,planes and speed boats and small subs.
But for large subs 2 to 7 plus feet there is nothing better than sealed lead acid batteries or Nimh packs,there ease of use and the fact that the SLAs can run in the wet and all the sizes and voltages available make them perfect for those subs.
Long run times ,verey inexpensive ,no special charger and they also serve to counterballance the sub.Instead of adding dead weight to counter ballance might as well be usefull power.
No safety concerns as bad as Lipos have and if you run them too far down lets say in an emergency return to port ,well there toast and alot of bucks down the drain!.
I have run SLAs in water and in the casing for years now ,just dont charge them sealed in!
For the money and for large subs there the ticket,that if you want the best bang for your buck and still have bucks left over
And no nueclear meltdown with fire,it sounds like the Alfa story ,they had to have the reactor running all the time or the bisthmus metal cooling fluid would solidify ,ruining the reactor/sub ouch not good engineering.
My OTW Type VIIc ,Oscar II ,Typhoon,Type XI,Type XXIII OTW,U-32,Octura,SB-2,Galatheand Grupner Shark and ARk M Red Shark all use or will use SLAs and NIMH,most of these subs have 2 plus hour run times.