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[Subculture]

A four wire cable would be rather chunky and difficult to hide, unless you use litz wire, which would probably be rather fragile unless strain relieved in some way.

Perhaps the simplest method would be to forget the electronics altogether, and use simple mechanical dynamic diving in conjunction with a tether.

Make a shark out of something buoyant like polystyrene foam or even balsa wood coated in resin, but construct the pectoral fins from something that can be bent like aluminium sheet. Bend them with a down angle, and ballast the shark down to a fairly low waterline with lead shot/sheet. When towed from behind the boat, the shark should dive, going deeper with increasing speed. The length of tether can control the maximum depth. The correct angle for the pectorals would need some experimentation via empirical tests. When the boat stops the shark should gradually rise to the surface, assuming the positive buoyancy isn't too great- else it'll pop to the surface like a cork.

[eddiesolo]

Could something like this do as the body: http://www.ebay.co.uk/itm/25cms-soft-touch-PLASTIC-WHALE-SHARK-seaside-life-sea-ocean-NEW-/130851980203?pt=UK_Toys_Creative_Educational_RL&hash=item1e7762c3ab
I know it is a whale shark, but size wise with a bit of fettling, also as it is soft you could split it and then do a dynamic dive model without electronics...just an idea.
Found this: http://www.ebay.co.uk/itm/Bullyland-Great-White-Shark-1-30-Scale-Model-16-CM-Brand-New-/370657688400?pt=UK_Collectables_AnimalCollectables_SM&hash=item564ceb7b50
Si:)

[red181]

thanks for suggestions, I suppose the budget is unreasonable, I didnt realise how expensive sub kit is, the best alternative is an mmb subsonic at £90 plus running gear, hence thinking a faulty, or batterred second hand unit should come in for less.
 
With regard to towing, I have tried this several times, I have one of those rc sharks tesco and others where selling some time ago. I towed using an old coathanger, so when the boat stopped, the shark didnt keep headbutting the boat, and string fouling the prop. The wire was fitted under the hull, so not really visible, and pivoted, allowing the shark to raise and fall. The pectorals where flybar paddles from an rc  helicopter, and could be angled to achieve raising when the boat moved, and sinking when stopped, I used a long bolt to act as a keel to keep the shark upright, it always wants to roll, but even though it worked, to a point, it made the model  a bit of a toy  in  a way, so I still fancy an rc version. pictures was an early version with clear leaxn to act as a plane  to create diving, unsuccessfully!  
 
Some time ago, there was a post with a "fin" sailing, it was really good, it was an upturned yatch hull, but I have no idea if it was capable of diving, I doubt it, so still thinking about this project, keep the ideas coming, the wires are a neat idea, but I suppose they would float, and there is the risk of fouling, the towed shark I had kept dragging all the 'rubbish' around in the local lakes!