Run DC light submerged in water?
Even if you use deionized water (which is a great insulator) it won't STAY a great insulator if you expose it to copper, brass, etc. in unsealed connections, which will leach ions and cause the water not to be deionized, nor a very good insulator. This will cause the lights in the water to fail, and may also offend your power supply.

If you want liquid, for some reason, mineral oil (USP) is clear, non-toxic, and a good electrical insulator that will stay an insulator. It will also make a mess if you tip the lamp over.

I'd suggest pouring the bottle full of epoxy, which will harden so it can't spill, and is an excellent insulator as well. Alternatively, find a size of marble or glass bead that fits through the neck opening and fill with those.