did some reading on various battery chemistry a while back and it appears to me - and i could be wrong - that the various flavors of lithium batts are not the best choice for this application for a few reasons...
* when you compare the lifespan and the time vs capacity loss curves for lithium vs nicad vs nimh, lithium starts looking a LOT less attractive
* very sensitive to over and under charging
* expensive and special charging equipment required, including balancers for multi-cell packs
* the high energy density/low weight doesn't do much for a submersible
of course using disposable batts is an extremely poor option form both environmental and wallet perspectives
in the mean time, NiMh tech keeps getting better, they have a decent track record, aren't fussy about charging and are less costly
also, changing to rechargeable batts would help solve the battery tube end cap problems and complexity - no springs or contacts needed, and the tubes could actually be permanently sealed with standard glue-on end caps. replacing packs on a job also becomes less of a hassle - un-do a connector, slide the tubes out, slide new ones in