Dear Andy,
Thankyou for your Post. Interesting and quite right.
I have just this moment come off the phone with my physicist friend (recently retired Lecturer/Research Physicist, now doing geophysics research consultancy) - opportune as the Univ. goes back tomorrow so bagged him at home! As usual when asking a tech. question/query of him, I had the normal fascinating and invaluable Lecture... so, what was intended to be only a five minute call lasted over half an hour! - enjoyed every minute!
As you and Malc(Fray) point out, the weight does indeed decrease the further a weight is immersed in water, and once fully immersed, any given weight is indeed proportionately less. This applies whether the weight is on the seabed or suspended in water, since displacement is pertinent whatever be the case. Gravity plays it's part in giving all/everything it's weight obviously, so one must be careful in measuring such an experiment since there are gravitational differences (local and gen.), around the planet - only small of course, but would affect fine measurement all the same, and need computations done to adjust for localised gravitational variances. This is all a bit OTT for our purposes I suppose, but if we are talking about it we may as well get it all as 'right' as poss.! - similarly as your interesting Post.
Horst raised other 'sideline' factors, this being that the 'power' needed to 'accelerate' 1 ton(shift, drag, or move), would be proportionately the same anywhere - whether on the moon, underwater, or anywhere else. This all down to mass and gravity equations. There is obviously more to this, and I have a page of notes from the conv. and my mind is buzzing trying to remember other things he said... but I think we have gone into fine techs. far enough now!!!
- and I must do some work this am.!!!
As to my 'ship-in-ship' thing, he saw something else against it connected with the 'free water' within, so I am going away to think on this when-&-as over the coming months!!!
Thankyou to all who have taken the time to participate - certainly helped blow away the Festive cobwebs!!!!!!
Regards, Bernard