If you want to avoid flashing light down just put a plastic disc underneath the LED's it'll shield the light from going straight down. 2 AA batteries will last quiet a bit longer than a joule thief on 1 and creating enough bouancy to float 2AA batteries should be trivial, a small block of wood. In fact the block of wood itself could be your bouy, just make it a disc about 6 inches in diameter and mount the LED's very close to the base in the middle, mount the batteries in a sealed container on a hollow rod about 6-8 inchs long underneath the disc to keep the bouy righted, might as well put the battery weight to use.