which Eveready 6-Volt Floating Hand Lantern are you talking about??? there are few, show us a pic. can't really tell you how to waterproof it without knowing that.
i have Roar of the Dolphin, i opened a 6v lantern battery case, (but don't try it with duracell alkaline 6v battery, it doesn't really have a case), removed 4 big cells, brake away divides, and put in any amount\type of cells you need or can fit.
for ROD you need up to 9,6v, (the more v the hotter bulb gets\faster burns out)
http://i61.photobucket.com/albums/h75/pike444/bulbs/3854-LROP.jpg
i used nicd sub c cells, just cuz i had them, if i had to buy cells i,d go li ion.
there is enough room in the case to fit up to 14 sub c cells, or unknown to me amount of li ion cells, i never tried li ion cells) for ROD you would need to wire 2p7s, i never tried 7cells with rop low 11w, i tried 6, i have no clue how plastic bulb holder will behave when heated up with 7 or more cells, but with 6 it works fine, stays solid, doesn't melt, i also tied and use 5cells magnumstar bulb with 6 cells, 7,4v. not as bright as rop low but a lot better than stock.
you can also fit D,C,AA...etc , almost any type of rechargadle cell
this can be done with any lantern, for the most part they have plastic bulbholder, so you don't wanna overheat it.
throw in these lanterns depend mostly on size\ shape of reflector, (besides brightness of the bulb) dolphin has large well shaped reflector, thus throws far.
if you need a light for 15ft deep, most likely lantern isn't for you, most are waterproofed just enough so they can float. idk which lantern you're talking about, but some are so crappy, not worth improving in any way, others can be improved, or might already be ok for 15ft.
