Please post your comments and suggestions...... 
Most any of them will probably work the same in the end. You probably need to further research the mix of "RF/wireless" and "underwater".
What does ROV mean?
Maybe you can also consider NRF24L01 modules (version with amp, not the small ones). I have a couple of those and they work very well above water, no idea how they perform underwater 
All of the methods mentioned in your unnecessary poll will work as badly as any other.
Bajdi:
What does ROV mean?
Remote Operated Vehicle
Typically only applied as terminology to underwater vehicles; for other vehicles, UAV = "Unmanned Aerial Vehicle", and UGV = "Unmanned Ground Vehicle".
There are probably other acronyms used as well (and probably different ones for other languages - the above are mainly for US english). Something I have always found strange, though, is why the english acronym that could be applied to -all- such vehicles became attached to underwater only vehicles; probably something historical or such.
Oh - and another word for similar vehicles (that is, remotely-controlled unmanned robotic vehicles): "telechir" - this word is, however, likely only found in UK literature (and possibly historical, at that - I own a book published in the UK called "Robots and Telechirs" by M.W. Thring, which was published in 1983; it's the only book I've read that has referred to these machines as such).
Telechir is presumably a latin portmanteau word, like television. Tele translates to 'remote' or 'distant', and I believe chir translates to 'hand', as in chiropractor. So telechir becomes 'remote/distant hand', which makes sense.
hi
sorry but you can not use RF system they don't work underwater, good system is rs485 cable
stefano
dxw00d:
Telechir is presumably a latin portmanteau word, like television. Tele translates to 'remote' or 'distant', and I believe chir translates to 'hand', as in chiropractor. So telechir becomes 'remote/distant hand', which makes sense.
I believe that was explained somewhere in the book; I probably should have pointed it out...
What I found interesting is that I have only seen that word used in that book, and occasionally in a few other works; IIRC, all were from the UK, as well. It seems like it might have been a coined word "from across the pond" that never really caught on (or stayed within the UK and/or Europe; perhaps it didn't catch "over here"?).
Is a telechir what is known as a waldo in the US?
AWOL:
Is a telechir what is known as a waldo in the US?
Well, the term "waldo" is kinda slang (IMHO); it also tends to be applied where "puppets" or other animated systems are concerned (like used by Disney and Lucasfilm - whether as a motion capture system, or to control animatronic systems).
What I believe is the industry term is "teleoperation" for these systems (ie, "teleoperated robot"); otherwise, when referring to individual systems, the terms "ROV" (underwater), UAV (flying), and UGV (ground) are used (there is also a specific acronym used for water surface craft, but I forget what it is; such craft tend to not be in as common usage as the other three).
There are wireless underwater technologies but industry hasn't picked up on them for use in rovs because of cost and the fact that there are too many unknown factors in any given body of water.
wired technologies are the most realistic option for most hobbyists.
What are you looking to build and what tasks will this equipment be expected to accomplish?
These are the ones I play with daily..... http://www.oceaneering.com/rovs/
The term "waldo" comes from a Robert A Heinlien story.
Waldo was a wealthy guy that had muscular dystrophy, he lived in an orbital habitat.
Just as a general statement: The lower the frequency, the better, but there can be additional factors as well.
Have a look here to see what kind of challenges are involved with this: Communication with submarines - Wikipedia
But as the others have said, It's not very feasible, and you won't be able to achieve much depth.
Hi
Don't even try.. Been there ...
Max depth in sea water 50cm max in fresh water 5 meters max.
As stated in post above the lower the frequency the better.
Use a umblicle cable.
Regards Antony
I think Elvis has left the building...