Any technically competent knows that EM waves (at communication frequencies) don't penetrate water very well.
Subs can't use it for communication, for example. So we know it can't penetrate for more than a few kilometers.
The pill didn't have much battery room, so had to use a low power transmitter, and a *very* tiny antenna, so understandably has short range.
But the issue was, how far could it penetrate with a normal antenna/battery? What if you had a RC sub that only went 2 feet deep, could it penetrate just 2 feet? My robot fish will operate in shallow water, not far from the receiver. I figured with 100m in air bluetooth range, I could at least get 2 feet underwater range. I also operate in de-ionized water which allows em waves to travel about 10x further than in sea water.
This test was to quantitatively define penetration range - less than a foot with a decent transmitter/antenna at 2.4GHz in pure water.
I'm finishing up some calculations on penetration depth vs transmission frequency. According to the math, dropping the frequency to 400MHz doesn't seem to help much - it only doubles the penetration depth to about ~2 feet. I'll run some experiments this week or next. I just need to squeeze out 4-5 feet depth and I'm good to go!
Oh and my math states that EM reflection off the water surface is negligible compared to absorption by water, ruling out various antenna tricks.